diff --git a/collects/planet/config.ss b/collects/planet/config.ss
index f444677cd8..cb8f3a1849 100644
--- a/collects/planet/config.ss
+++ b/collects/planet/config.ss
@@ -6,7 +6,9 @@
     (PLANET-SERVER-NAME       "planet.plt-scheme.org")
     (PLANET-SERVER-PORT       270)
     (PLANET-CODE-VERSION      "300")
-    (PLANET-DIR               (build-path (find-system-path 'addon-dir) "planet" (PLANET-CODE-VERSION)))
+    (PLANET-DIR               (if (getenv "PLTPLANETDIR")
+                                  (string->path (getenv "PLTPLANETDIR"))
+                                  (build-path (find-system-path 'addon-dir) "planet" (PLANET-CODE-VERSION))))
     (CACHE-DIR                (build-path (PLANET-DIR) "cache"))
     (LINKAGE-FILE             (build-path (PLANET-DIR) "LINKAGE"))
     (LOGGING-ENABLED?         #t)
diff --git a/collects/planet/doc.txt b/collects/planet/doc.txt
index a326bae7c5..3261f423e9 100644
--- a/collects/planet/doc.txt
+++ b/collects/planet/doc.txt
@@ -52,8 +52,9 @@ Parameters:
 > (PLANET-DIR) -> directory-string
 > (PLANET-DIR directory-string) -> void
 
-The root PLaneT directory. Default is the directory in which config.ss
-is found.
+The root PLaneT directory. If the environment variable PLTPLANETDIR is
+set, default is its value; otherwise the default is the directory in
+which config.ss is found.
 
 > (CACHE-DIR) -> directory-string
 > (CACHE-DIR directory-string) -> void