diff --git a/LOG b/LOG
index b9f706b8b8..1e08bd1a35 100644
--- a/LOG
+++ b/LOG
@@ -523,3 +523,7 @@
     c/Mf-*
 - Suppress warnings from implicit fallthrough in case labels.
     Mf-{a6,arm32,i3,ppc,ta6,ti3,tpp32}le
+- added bytevector-compress and bytevector-uncompress
+    bytevector.ss, primdata.ss, new-io.c, prim5.c, externs.h,
+    objects.stex, release_notes.stex,
+    bytevector.ms, root-experr*
diff --git a/c/externs.h b/c/externs.h
index c554ec1e5f..8014632f5b 100644
--- a/c/externs.h
+++ b/c/externs.h
@@ -197,6 +197,12 @@ extern ptr S_get_fd_length PROTO((ptr file, IBOOL gzflag));
 extern ptr S_set_fd_length PROTO((ptr file, ptr length, IBOOL gzflag));
 extern void S_new_io_init PROTO((void));
 
+extern uptr S_bytevector_compress_size PROTO((iptr s_count));
+extern ptr S_bytevector_compress PROTO((ptr dest_bv, iptr d_start, iptr d_count,
+                                        ptr src_bv, iptr s_start, iptr s_count));
+extern ptr S_bytevector_uncompress PROTO((ptr dest_bv, iptr d_start, iptr d_count,
+                                          ptr src_bv, iptr s_start, iptr s_count));
+
 /* thread.c */
 extern void S_thread_init PROTO((void));
 extern ptr S_create_thread_object PROTO((const char *who, ptr p_tc));
diff --git a/c/new-io.c b/c/new-io.c
index 6f1ce810d1..a3d3c48222 100644
--- a/c/new-io.c
+++ b/c/new-io.c
@@ -54,6 +54,7 @@ static ptr new_open_output_fd_helper PROTO((const char *filename, INT mode,
              INT append, INT lock, INT replace, INT compressed));
 static INT lockfile PROTO((INT fd));
 static ptr make_gzxfile PROTO((int fd, gzFile file));
+static int is_valid_zlib_length(iptr count);
 
 /*
  not_ok_is_fatal: !ok definitely implies error, so ignore gzerror
@@ -783,3 +784,65 @@ void S_new_io_init() {
   _setmode(_fileno(stderr), O_BINARY);
 #endif /* WIN32 */
 }
+
+static int is_valid_zlib_length(iptr count) {
+  /* A zlib `uLong` may be the same as `unsigned long`,
+     which is not as big as `iptr` on 64-bit Windows. */
+  return count == (iptr)(uLong)count;
+}
+
+/* Accept `iptr` because we expect it to represent a bytevector size,
+   which always fits in `iptr`. Return `uptr`, because the result might
+   not fit in `iptr`. */
+uptr S_bytevector_compress_size(iptr s_count) {
+  if (is_valid_zlib_length(s_count))
+    return compressBound(s_count);
+  else {
+    /* Compression will report "source too long" */
+    return 0;
+  }
+}
+
+ptr S_bytevector_compress(ptr dest_bv, iptr d_start, iptr d_count,
+                          ptr src_bv, iptr s_start, iptr s_count) {
+  /* On error, an message-template string with ~s for the bytevector */
+  int r;
+  uLong destLen;
+
+  if (!is_valid_zlib_length(s_count))
+    return Sstring("source bytevector ~s is too large");
+
+  destLen = d_count;
+
+  r = compress(&BVIT(dest_bv, d_start), &destLen, &BVIT(src_bv, s_start), s_count);
+
+  if (r == Z_OK)
+    return FIX(destLen);
+  else if (r == Z_BUF_ERROR)
+    return Sstring("destination bytevector is too small for compressed form of ~s");
+  else
+    return Sstring("internal error compressing ~s");
+}
+
+ptr S_bytevector_uncompress(ptr dest_bv, iptr d_start, iptr d_count,
+                            ptr src_bv, iptr s_start, iptr s_count) {
+  /* On error, an message-template string with ~s for the bytevector */
+  int r;
+  uLongf destLen;
+
+  if (!is_valid_zlib_length(d_count))
+    return Sstring("expected result size of uncompressed source ~s is too large");
+
+  destLen = d_count;
+
+  r = uncompress(&BVIT(dest_bv, d_start), &destLen, &BVIT(src_bv, s_start), s_count);
+
+  if (r == Z_OK)
+    return FIX(destLen);
+  else if (r == Z_BUF_ERROR)
+    return Sstring("uncompressed ~s is larger than expected size");
+  else if (r == Z_DATA_ERROR)
+    return Sstring("invalid data in source bytevector ~s");
+  else
+    return Sstring("internal error uncompressing ~s");
+}
diff --git a/c/prim5.c b/c/prim5.c
index 0cffbc2189..5bee635c00 100644
--- a/c/prim5.c
+++ b/c/prim5.c
@@ -1554,6 +1554,10 @@ void S_prim5_init() {
     Sforeign_symbol("(cs)get_fd_length", (void*)S_get_fd_length);
     Sforeign_symbol("(cs)set_fd_length", (void*)S_set_fd_length);
 
+    Sforeign_symbol("(cs)bytevector_compress_size", (void*)S_bytevector_compress_size);
+    Sforeign_symbol("(cs)bytevector_compress", (void*)S_bytevector_compress);
+    Sforeign_symbol("(cs)bytevector_uncompress", (void*)S_bytevector_uncompress);
+
     Sforeign_symbol("(cs)logand", (void *)S_logand);
     Sforeign_symbol("(cs)logbitp", (void *)S_logbitp);
     Sforeign_symbol("(cs)logbit0", (void *)S_logbit0);
diff --git a/csug/objects.stex b/csug/objects.stex
index 0d4e5569e4..d243e7107a 100644
--- a/csug/objects.stex
+++ b/csug/objects.stex
@@ -1129,7 +1129,7 @@ Negative values are stored as their two's complement equivalent.
 %----------------------------------------------------------------------------
 \entryheader
 \formdef{bytevector->immutable-bytevector}{\categoryprocedure}{(bytevector->immutable-bytevector \var{bytevector})}
-\returns an immutable \var{bytevector} equal to \var{bytevector}
+\returns an immutable bytevector equal to \var{bytevector}
 \listlibraries
 \endentryheader
 
@@ -1144,6 +1144,33 @@ is immutable; otherwise, the result is an immutable bytevector with the same con
 \endschemedisplay
 
 
+%----------------------------------------------------------------------------
+\entryheader
+\formdef{bytevector-compress}{\categoryprocedure}{(bytevector-compress \var{bytevector})}
+\returns a new bytevector containing compressed content of \var{bytevector}
+\listlibraries
+\endentryheader
+
+\noindent
+The result is the raw compressed data with a minimal header to record
+the uncompressed size and the compression mode. The result does not include
+the header that is written by port-based compression using the
+\scheme{compressed} option.
+
+
+%----------------------------------------------------------------------------
+\entryheader
+\formdef{bytevector-uncompress}{\categoryprocedure}{(bytevector-uncompress \var{bytevector})}
+\returns a bytevector containing uncompressed content of \var{bytevector}
+\listlibraries
+\endentryheader
+
+\noindent
+Uncompresses a \var{bytevector} produced by
+\scheme{bytevector-compress} to a new bytevector with the same content
+as the original given to \scheme{bytevector-compress}.
+
+
 \section{Boxes\label{SECTBOXES}}
 
 \index{boxes}Boxes are single-cell objects that are primarily useful for providing
diff --git a/mats/bytevector.ms b/mats/bytevector.ms
index e1492a5c35..61709e76d6 100644
--- a/mats/bytevector.ms
+++ b/mats/bytevector.ms
@@ -11011,3 +11011,45 @@
     (number? (bytevector-ieee-single-native-ref immutable-100-bytevector 0))
     (number? (bytevector-ieee-double-native-ref immutable-100-bytevector 0))
 )
+
+
+(mat bytevector-compress
+    (error? (bytevector-compress 7))
+    (error? (bytevector-compress "hello"))
+    (error? (bytevector-uncompress 7))
+    (error? (bytevector-uncompress "hello"))
+    (begin
+      (define (round-trip-bytevector-compress bv)
+        (equal? (bytevector-uncompress (bytevector-compress bv))
+                bv))
+      (round-trip-bytevector-compress (string->utf8 "hello")))
+    (round-trip-bytevector-compress '#vu8())
+    (round-trip-bytevector-compress (apply bytevector
+                                           (let loop ([i 0])
+                                             (if (= i 4096)
+                                                 '()
+                                                 (cons (bitwise-and i 255)
+                                                       (loop (+ i 1)))))))
+    (error?
+     ;; Need at least 8 bytes for result size
+     (bytevector-uncompress '#vu8()))
+    (error?
+     ;; Need at least 8 bytes for result size
+     (bytevector-uncompress '#vu8(0 0 0 0 0 0 255)))
+    (error?
+     ;; Fail if the uncompressed result is too big
+     (bytevector-uncompress (let ([bv (bytevector-compress (string->utf8 "hello"))])
+                              (bytevector-u64-set! bv 0 (sub1 (bytevector-u64-ref bv 0 (endianness big))) (endianness big))
+                              bv)))
+    (error?
+     ;; Fail if the uncompressed result is too small
+     (bytevector-uncompress (let ([bv (bytevector-compress (string->utf8 "hello"))])
+                              (bytevector-u64-set! bv 0 (add1 (bytevector-u64-ref bv 0 (endianness big)))  (endianness big))
+                              bv)))
+    (error?
+     ;; Compressed data always starts with 0x78, so this one isn't valid:
+     (bytevector-uncompress '#vu8(0 0 0 0 0 0 0 255 1 2 3)))
+    (error?
+     ;; Claming a too-large size in the header should fail with a suitable message:
+     (bytevector-uncompress '#vu8(255 255 255 255 255 255 255 255 1 2 3)))
+)
diff --git a/mats/root-experr-compile-0-f-f-f b/mats/root-experr-compile-0-f-f-f
index 71767263fb..503bbb6127 100644
--- a/mats/root-experr-compile-0-f-f-f
+++ b/mats/root-experr-compile-0-f-f-f
@@ -3573,6 +3573,16 @@ bytevector.mo:Expected error in mat bytevector->immutable-bytevector: "bytevecto
 bytevector.mo:Expected error in mat bytevector->immutable-bytevector: "bytevector-fill!: #vu8(42 42 42 42 42 42 ...) is not a mutable bytevector".
 bytevector.mo:Expected error in mat bytevector->immutable-bytevector: "bytevector-copy!: #vu8(42 42 42 42 42 42 ...) is not a mutable bytevector".
 bytevector.mo:Expected error in mat bytevector->immutable-bytevector: "bytevector-truncate!: #vu8(42 42 42 42 42 42 ...) is not a mutable bytevector".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-compress: 7 is not a bytevector".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-compress: "hello" is not a bytevector".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: 7 is not a bytevector".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: "hello" is not a bytevector".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: invalid data in source bytevector #vu8()".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: invalid data in source bytevector #vu8(0 0 0 0 0 0 ...)".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: uncompressed #vu8(0 0 0 0 0 0 ...) is larger than expected size".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: uncompressed size 5 for #vu8(0 0 0 0 0 0 ...) is smaller than expected size 6".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: invalid data in source bytevector #vu8(0 0 0 0 0 0 ...)".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: bytevector #vu8(255 255 255 255 255 255 ...) claims invalid uncompressed size <int>".
 misc.mo:Expected error in mat compiler1: "variable i-am-not-bound is not bound".
 misc.mo:Expected error in mat compiler1: "attempt to apply non-procedure oops".
 misc.mo:Expected error in mat compiler1: "incorrect argument count in call (g (list))".
diff --git a/mats/root-experr-compile-2-f-f-f b/mats/root-experr-compile-2-f-f-f
index 71767263fb..503bbb6127 100644
--- a/mats/root-experr-compile-2-f-f-f
+++ b/mats/root-experr-compile-2-f-f-f
@@ -3573,6 +3573,16 @@ bytevector.mo:Expected error in mat bytevector->immutable-bytevector: "bytevecto
 bytevector.mo:Expected error in mat bytevector->immutable-bytevector: "bytevector-fill!: #vu8(42 42 42 42 42 42 ...) is not a mutable bytevector".
 bytevector.mo:Expected error in mat bytevector->immutable-bytevector: "bytevector-copy!: #vu8(42 42 42 42 42 42 ...) is not a mutable bytevector".
 bytevector.mo:Expected error in mat bytevector->immutable-bytevector: "bytevector-truncate!: #vu8(42 42 42 42 42 42 ...) is not a mutable bytevector".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-compress: 7 is not a bytevector".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-compress: "hello" is not a bytevector".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: 7 is not a bytevector".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: "hello" is not a bytevector".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: invalid data in source bytevector #vu8()".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: invalid data in source bytevector #vu8(0 0 0 0 0 0 ...)".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: uncompressed #vu8(0 0 0 0 0 0 ...) is larger than expected size".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: uncompressed size 5 for #vu8(0 0 0 0 0 0 ...) is smaller than expected size 6".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: invalid data in source bytevector #vu8(0 0 0 0 0 0 ...)".
+bytevector.mo:Expected error in mat bytevector-compress: "bytevector-uncompress: bytevector #vu8(255 255 255 255 255 255 ...) claims invalid uncompressed size <int>".
 misc.mo:Expected error in mat compiler1: "variable i-am-not-bound is not bound".
 misc.mo:Expected error in mat compiler1: "attempt to apply non-procedure oops".
 misc.mo:Expected error in mat compiler1: "incorrect argument count in call (g (list))".
diff --git a/release_notes/release_notes.stex b/release_notes/release_notes.stex
index d2a50b6fd9..b484eab0a2 100644
--- a/release_notes/release_notes.stex
+++ b/release_notes/release_notes.stex
@@ -105,6 +105,13 @@ For example, the arity mask for a two-argument procedure such as
 while the arity mask for a procedure that accepts one or more arguments,
 such as \var{list*}, is $-2$ (all but bit 0 set).
 
+\subsection{Bytevector compression (9.4.1)}
+
+The new primitive procedures \scheme{bytevector-compress} and
+\scheme{bytevector-decompress} exposes for bytevectors the kind of
+compression functionality that is used for files with the
+\scheme{compressed} option.
+
 \subsection{High-precision clock time in Windows 8 and up (9.4.1)}
 
 When running on Windows 8 and up, Chez Scheme uses the high-precision
diff --git a/s/bytevector.ss b/s/bytevector.ss
index 44bccfaeb4..c45c46e013 100644
--- a/s/bytevector.ss
+++ b/s/bytevector.ss
@@ -1452,4 +1452,63 @@
                    (list->little ls size)
                    (unrecognized-endianness who eness))]))))
   )
+
+  (let ()
+    ;; Store uncompressed size as u64:
+    (define uncompressed-length-length (ftype-sizeof integer-64))
+    ;; Always big-endian, so that compressed data is portable.
+    ;; It might be useful somehow that valid compressed data always starts
+    ;; with a 0 byte; otherwise, the expected size would be unrealistically big.
+    (define uncompressed-length-endianness (endianness big))
+
+    (define $bytevector-compress-size
+      (foreign-procedure "(cs)bytevector_compress_size" (iptr) uptr))
+    (define $bytevector-compress
+      (foreign-procedure "(cs)bytevector_compress" (scheme-object iptr iptr scheme-object iptr iptr) scheme-object))
+    (define $bytevector-uncompress
+      (foreign-procedure "(cs)bytevector_uncompress" (scheme-object iptr iptr scheme-object iptr iptr) scheme-object))
+
+    (set-who! bytevector-compress
+      (lambda (bv)
+        (unless (bytevector? bv) (not-a-bytevector who bv))
+        (let* ([dest-max-len ($bytevector-compress-size (bytevector-length bv))]
+               [dest-alloc-len (min (+ dest-max-len uncompressed-length-length)
+                                    ;; In the unlikely event of a non-fixnum requested size...
+                                    (constant maximum-bytevector-length))]
+               [dest-bv (make-bytevector dest-alloc-len)])
+          (let ([r ($bytevector-compress dest-bv
+                                         uncompressed-length-length
+                                         (fx- dest-alloc-len uncompressed-length-length)
+                                         bv
+                                         0
+                                         (bytevector-length bv))])
+            (cond
+             [(string? r)
+              ($oops who r bv)]
+             [else
+              ($bytevector-u64-set! dest-bv 0 (bytevector-length bv) uncompressed-length-endianness who)
+              (bytevector-truncate! dest-bv (fx+ r uncompressed-length-length))])))))
+
+    (set-who! bytevector-uncompress
+      (lambda (bv)
+        (unless (bytevector? bv) (not-a-bytevector who bv))
+        (unless (>= (bytevector-length bv) uncompressed-length-length)
+          ($oops who "invalid data in source bytevector ~s" bv))
+        (let ([dest-length ($bytevector-u64-ref bv 0 uncompressed-length-endianness who)])
+          (unless (and (fixnum? dest-length)
+                       ($fxu< dest-length (constant maximum-bytevector-length)))
+            ($oops who "bytevector ~s claims invalid uncompressed size ~s" bv dest-length))
+          (let* ([dest-bv (make-bytevector dest-length)]
+                 [r ($bytevector-uncompress dest-bv
+                                            0
+                                            dest-length
+                                            bv
+                                            uncompressed-length-length
+                                            (fx- (bytevector-length bv) uncompressed-length-length))])
+            (cond
+             [(string? r) ($oops who r bv)]
+             [(fx= r dest-length) dest-bv]
+             [else
+              ($oops who "uncompressed size ~s for ~s is smaller than expected size ~a" r bv dest-length)]))))))
+
 )
diff --git a/s/primdata.ss b/s/primdata.ss
index e331c003b7..e4e18e0d85 100644
--- a/s/primdata.ss
+++ b/s/primdata.ss
@@ -1160,6 +1160,8 @@
   (bytevector-u48-set! [sig [(bytevector sub-index symbol u48) -> (void)]] [flags true])
   (bytevector-u56-ref [sig [(bytevector sub-index symbol) -> (u56)]] [flags true mifoldable discard])
   (bytevector-u56-set! [sig [(bytevector sub-index symbol u56) -> (void)]] [flags true])
+  (bytevector-compress [sig [(ptr) -> (ptr)]] [flags])
+  (bytevector-uncompress [sig [(ptr) -> (ptr)]] [flags])
   (call/1cc [sig [(procedure) -> (ptr ...)]] [flags])
   (call-with-input-file [sig [(pathname procedure) (pathname procedure sub-ptr) -> (ptr ...)]] [flags ieee r5rs])      ; has options argument
   (call-with-output-file [sig [(pathname procedure) (pathname procedure sub-ptr) -> (ptr ...)]] [flags ieee r5rs])     ; has options argument