diff --git a/Documentation/git-write-tree.txt b/Documentation/git-write-tree.txt
index 77e12cb949..c85fa89c30 100644
--- a/Documentation/git-write-tree.txt
+++ b/Documentation/git-write-tree.txt
@@ -8,7 +8,7 @@ git-write-tree - Creates a tree object from the current index
 
 SYNOPSIS
 --------
-'git-write-tree' [--missing-ok]
+'git-write-tree' [--missing-ok] [--prefix=<prefix>/]
 
 DESCRIPTION
 -----------
@@ -30,6 +30,12 @@ OPTIONS
 	directory exist in the object database.  This option disables this
 	check.
 
+--prefix=<prefix>/::
+	Writes a tree object that represents a subdirectory
+	`<prefix>`.  This can be used to write the tree object
+	for a subproject that is in the named subdirectory.
+
+
 Author
 ------
 Written by Linus Torvalds <torvalds@osdl.org>
diff --git a/cache-tree.c b/cache-tree.c
index e452238ba7..dae4399c1e 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -525,3 +525,29 @@ struct cache_tree *cache_tree_read(const char *buffer, unsigned long size)
 		return NULL; /* not the whole tree */
 	return read_one(&buffer, &size);
 }
+
+struct cache_tree *cache_tree_find(struct cache_tree *it, const char *path)
+{
+	while (*path) {
+		const char *slash;
+		struct cache_tree_sub *sub;
+
+		slash = strchr(path, '/');
+		if (!slash)
+			slash = path + strlen(path);
+		/* between path and slash is the name of the
+		 * subtree to look for.
+		 */
+		sub = find_subtree(it, path, slash - path, 0);
+		if (!sub)
+			return NULL;
+		it = sub->cache_tree;
+		if (slash)
+			while (*slash && *slash == '/')
+				slash++;
+		if (!slash || !*slash)
+			return it; /* prefix ended with slashes */
+		path = slash;
+	}
+	return it;
+}
diff --git a/cache-tree.h b/cache-tree.h
index 72c64801f5..119407e3a1 100644
--- a/cache-tree.h
+++ b/cache-tree.h
@@ -28,4 +28,6 @@ struct cache_tree *cache_tree_read(const char *buffer, unsigned long size);
 int cache_tree_fully_valid(struct cache_tree *);
 int cache_tree_update(struct cache_tree *, struct cache_entry **, int, int, int);
 
+struct cache_tree *cache_tree_find(struct cache_tree *, const char *);
+
 #endif
diff --git a/t/t0000-basic.sh b/t/t0000-basic.sh
index cf33989b56..2c9bbb59b0 100755
--- a/t/t0000-basic.sh
+++ b/t/t0000-basic.sh
@@ -195,6 +195,20 @@ test_expect_success \
     'git-ls-tree -r output for a known tree.' \
     'diff current expected'
 
+test_expect_success \
+    'writing partial tree out with git-write-tree --prefix.' \
+    'ptree=$(git-write-tree --prefix=path3)'
+test_expect_success \
+    'validate object ID for a known tree.' \
+    'test "$ptree" = 21ae8269cacbe57ae09138dcc3a2887f904d02b3'
+
+test_expect_success \
+    'writing partial tree out with git-write-tree --prefix.' \
+    'ptree=$(git-write-tree --prefix=path3/subp3)'
+test_expect_success \
+    'validate object ID for a known tree.' \
+    'test "$ptree" = 3c5e5399f3a333eddecce7a9b9465b63f65f51e2'
+
 ################################################################
 rm .git/index
 test_expect_success \
diff --git a/write-tree.c b/write-tree.c
index 7a4f691d8a..895e7a359d 100644
--- a/write-tree.c
+++ b/write-tree.c
@@ -8,8 +8,10 @@
 #include "cache-tree.h"
 
 static int missing_ok = 0;
+static char *prefix = NULL;
 
-static const char write_tree_usage[] = "git-write-tree [--missing-ok]";
+static const char write_tree_usage[] =
+"git-write-tree [--missing-ok] [--prefix=<prefix>/]";
 
 static struct cache_file cache_file;
 
@@ -21,13 +23,18 @@ int main(int argc, char **argv)
 
 	newfd = hold_index_file_for_update(&cache_file, get_index_file());
 	entries = read_cache();
-	if (argc == 2) {
-		if (!strcmp(argv[1], "--missing-ok"))
+
+	while (1 < argc) {
+		char *arg = argv[1];
+		if (!strcmp(arg, "--missing-ok"))
 			missing_ok = 1;
+		else if (!strncmp(arg, "--prefix=", 9))
+			prefix = arg + 9;
 		else
 			die(write_tree_usage);
+		argc--; argv++;
 	}
-	
+
 	if (argc > 2)
 		die("too many options");
 
@@ -54,6 +61,12 @@ int main(int argc, char **argv)
 		 * performance penalty and not a big deal.
 		 */
 	}
-	printf("%s\n", sha1_to_hex(active_cache_tree->sha1));
+	if (prefix) {
+		struct cache_tree *subtree =
+			cache_tree_find(active_cache_tree, prefix);
+		printf("%s\n", sha1_to_hex(subtree->sha1));
+	}
+	else
+		printf("%s\n", sha1_to_hex(active_cache_tree->sha1));
 	return 0;
 }