diff --git a/doc/changes/9827.md b/doc/changes/9827.md
new file mode 100644
index 00000000000..bad7a3ec485
--- /dev/null
+++ b/doc/changes/9827.md
@@ -0,0 +1,2 @@
+- add `only_sources` field to `copy_files` stanza (#9827, fixes #9709,
+  @jchavarri)
diff --git a/doc/stanzas/copy_files.rst b/doc/stanzas/copy_files.rst
index 7279a6bfa9b..8f1c274bdcb 100644
--- a/doc/stanzas/copy_files.rst
+++ b/doc/stanzas/copy_files.rst
@@ -25,6 +25,9 @@ details.
 - ``(enabled_if <blang expression>)`` conditionally disables this stanza. The
   condition is specified using the :doc:`reference/boolean-language`.
 
+- ``(only_sources <blang expression>)`` specifies that the glob in ``files``
+  gets applied over the source tree, and not the build tree.
+
 The short form:
 
 .. code:: dune
diff --git a/src/dune_rules/simple_rules.ml b/src/dune_rules/simple_rules.ml
index 135102e7f58..63e99b54be4 100644
--- a/src/dune_rules/simple_rules.ml
+++ b/src/dune_rules/simple_rules.ml
@@ -218,7 +218,15 @@ let copy_files sctx ~dir ~expander ~src_dir (def : Copy_files.t) =
            not the current directory."
       ];
   (* add rules *)
-  let* files = Build_system.eval_pred (File_selector.of_glob ~dir:src_in_build glob) in
+  let* only_sources = Expander.eval_blang expander def.only_sources in
+  let* files =
+    let dir =
+      match only_sources with
+      | true -> src_in_src
+      | false -> src_in_build
+    in
+    Build_system.eval_pred (File_selector.of_glob ~dir glob)
+  in
   (* CR-someday amokhov: We currently traverse the set [files] twice: first, to
      add the corresponding rules, and then to convert the files to [targets]. To
      do only one traversal we need [Memo.parallel_map_set]. *)
diff --git a/src/dune_rules/stanzas/copy_files.ml b/src/dune_rules/stanzas/copy_files.ml
index 5c3a92167ca..d4085cd7271 100644
--- a/src/dune_rules/stanzas/copy_files.ml
+++ b/src/dune_rules/stanzas/copy_files.ml
@@ -1,12 +1,17 @@
 open Import
 open Dune_lang.Decoder
 
+type origin =
+  | Build
+  | Source
+
 type t =
   { add_line_directive : bool
   ; alias : Alias.Name.t option
   ; mode : Rule.Mode.t
   ; enabled_if : Blang.t
   ; files : String_with_vars.t
+  ; only_sources : Blang.t
   ; syntax_version : Dune_lang.Syntax.Version.t
   }
 
@@ -16,14 +21,33 @@ include Stanza.Make (struct
     include Poly
   end)
 
+let decode_only_sources =
+  let* blang = peek in
+  match blang with
+  | Some _ -> Blang.decode
+  | None -> return Blang.true_
+;;
+
 let long_form =
   let check = Dune_lang.Syntax.since Stanza.syntax (2, 7) in
   let+ alias = field_o "alias" (check >>> Dune_lang.Alias.decode)
   and+ mode = field "mode" ~default:Rule.Mode.Standard (check >>> Rule_mode_decoder.decode)
   and+ enabled_if = Enabled_if.decode ~allowed_vars:Any ~since:(Some (2, 8)) ()
   and+ files = field "files" (check >>> String_with_vars.decode)
+  and+ only_sources =
+    field_o
+      "only_sources"
+      (Dune_lang.Syntax.since Stanza.syntax (3, 14) >>> decode_only_sources)
   and+ syntax_version = Dune_lang.Syntax.get_exn Stanza.syntax in
-  { add_line_directive = false; alias; mode; enabled_if; files; syntax_version }
+  let only_sources = Option.value only_sources ~default:Blang.false_ in
+  { add_line_directive = false
+  ; alias
+  ; mode
+  ; enabled_if
+  ; files
+  ; only_sources
+  ; syntax_version
+  }
 ;;
 
 let decode =
@@ -38,6 +62,7 @@ let decode =
     ; mode = Standard
     ; enabled_if = Blang.true_
     ; files
+    ; only_sources = Blang.false_
     ; syntax_version
     }
 ;;
diff --git a/src/dune_rules/stanzas/copy_files.mli b/src/dune_rules/stanzas/copy_files.mli
index 1702735a099..16d5e383994 100644
--- a/src/dune_rules/stanzas/copy_files.mli
+++ b/src/dune_rules/stanzas/copy_files.mli
@@ -1,11 +1,16 @@
 open Import
 
+type origin =
+  | Build
+  | Source
+
 type t =
   { add_line_directive : bool
   ; alias : Alias.Name.t option
   ; mode : Rule.Mode.t
   ; enabled_if : Blang.t
   ; files : String_with_vars.t
+  ; only_sources : Blang.t
   ; syntax_version : Dune_lang.Syntax.Version.t
   }
 
diff --git a/test/blackbox-tests/test-cases/copy_files/test6.t b/test/blackbox-tests/test-cases/copy_files/test6.t
new file mode 100644
index 00000000000..9c8dc1d541a
--- /dev/null
+++ b/test/blackbox-tests/test-cases/copy_files/test6.t
@@ -0,0 +1,45 @@
+Show that copy_files operates on the build folder
+
+  $ mkdir -p target foo
+  $ cat >dune-project <<EOF
+  > (lang dune 3.14)
+  > EOF
+  $ cat >target/dune <<EOF
+  > (copy_files
+  >  (files ../foo/*.txt))
+  > EOF
+  $ cat >foo/dune <<EOF
+  > (rule
+  >  (write-file in-build.txt ""))
+  > EOF
+
+  $ touch foo/in-source.txt
+
+  $ dune build target/in-source.txt
+  $ dune build target/in-build.txt
+
+Show the difference when `only_sources` is used
+
+  $ cat >target/dune <<EOF
+  > (copy_files
+  >  (only_sources)
+  >  (files ../foo/*.txt))
+  > EOF
+
+  $ dune build target/in-source.txt
+  $ dune build target/in-build.txt
+  Error: Don't know how to build target/in-build.txt
+  [1]
+
+A blang expression can be used:
+
+  $ cat >target/dune <<EOF
+  > (copy_files
+  >  (only_sources (= x x))
+  >  (files ../foo/*.txt))
+  > EOF
+
+  $ dune build target/in-source.txt
+  $ dune build target/in-build.txt
+  Error: Don't know how to build target/in-build.txt
+  [1]