diff --git a/cli/zilch-ninja.scm b/cli/zilch-ninja.scm
index 3574953..ef0a74d 100644
--- a/cli/zilch-ninja.scm
+++ b/cli/zilch-ninja.scm
@@ -21,6 +21,7 @@
     (max-jobs #t #\j)
     (verbose #f #\v)
     (source #t #\s)
+    (project #t #\p)
     (print-build-logs #f #\L))
   (list->vector (cdr (command-line)))
   print-help))
@@ -64,6 +65,16 @@
   (call-with-output-file (ninja-build-config-depfile-path config) (lambda (p) (write (mapping->alist (finalized-drv-depfile finalized-drv)) p))))
  (for-each (lambda (v) (export-depfiles (force (drv-rewrite-finalized-drv v)))) (finalized-drv-rewritten-drvs finalized-drv)))
 
+(for-each
+ (lambda (c)
+  (when (eq? (car c) 'project)
+   (let loop ((rewrites (ninja-build-config-rewrites config)))
+    (cond
+     ((null? rewrites) (error "Subproject not found" (cdr c)))
+     ((string=? (caar rewrites) (cdr c)) (set! config (cdar rewrites)))
+     (else (loop (cdr rewrites)))))))
+ options)
+
 (cond
  ((string=? (car args) "source")
   (let*-values
diff --git a/docs/modules/ROOT/pages/ninja/man.adoc b/docs/modules/ROOT/pages/ninja/man.adoc
index d2da5da..17eaf51 100644
--- a/docs/modules/ROOT/pages/ninja/man.adoc
+++ b/docs/modules/ROOT/pages/ninja/man.adoc
@@ -41,6 +41,10 @@ written. See the documentation at https://puck.moe/zilch/docs/.
 *--help*::
   Print a help message.
 
+*-p* _NAME_::
+*--project* _NAME_::
+  Operate only on a rewritten input derivation.
+
 *-s* _DIR_::
 *--source* _DIR_::
   Override the input source for builds. Doesn't reconfigure the build,