diff --git a/lisp/progmodes/project.el b/lisp/progmodes/project.el
index 5b8648031fb..05ba631e52f 100644
--- a/lisp/progmodes/project.el
+++ b/lisp/progmodes/project.el
@@ -66,6 +66,9 @@
 ;; files, but supports additions to the list using the user option
 ;; `project-vc-ignores' (usually through .dir-locals.el).
 ;;
+;; At this point the name might as well be an abbreviation for "VC and
+;; Etc", see the variable `project-vc-extra-root-markers'.
+;;
 ;; Utils:
 ;;
 ;; `project-combine-directories' and `project-subtract-directories',
@@ -411,6 +414,51 @@ project-vc-name
   :version "29.1"
   :safe #'stringp)
 
+;; Not using regexps because these wouldn't work in Git pathspecs, in
+;; case we decide we need to be able to list subprojects.
+(defcustom project-vc-extra-root-markers nil
+  "List of additional markers to signal project roots.
+
+A marker is either a base file name or a glob pattern for such.
+
+A directory containing such a marker file or a file matching a
+marker pattern will be recognized as the root of a VC project.
+
+Example values: \".dir-locals.el\", \"package.json\", \"pom.xml\",
+\"requirements.txt\", \"Gemfile\", \"*.gemspec\", \"autogen.sh\".
+
+These will be used in addition to regular directory markers such
+as \".git\", \".hg\", and so on, depending on the value of
+`vc-handled-backends'.  It is most useful when a VC project has
+subdirectories inside it that need to be considered as separate
+projects.  It can also be used for projects outside of VC
+repositories.
+
+In either case, their behavior will still obey the relevant
+variables, such as `project-vc-ignores' or `project-vc-name'."
+  :type 'list
+  :version "29.1"
+  :safe (lambda (val) (and (listp val) (cl-every #'stringp val))))
+
+(defcustom project-vc-subprojects nil
+  "List of relative directory names to consider separate projects.
+Each entry should a string, name of a subproject root directory
+relative to the VC project root.
+
+Whenever a VC project root detected according to the usual
+conditions contains a subdirectory from that list, that
+subdirectory will be recognized as the root of a separate VC
+project as well.
+
+One would usually set this variable through the dir-locals
+mechanism.
+
+If subprojects are Git submodules, you can use the variable
+`project-vc-merge-submodules' instead."
+  :type 'list
+  :version "29.1"
+  :safe (lambda (val) (and (listp val) (seq-every-p #'stringp val))))
+
 ;; FIXME: Using the current approach, major modes are supposed to set
 ;; this variable to a buffer-local value.  So we don't have access to
 ;; the "external roots" of language A from buffers of language B, which
@@ -447,29 +495,59 @@ project-vc-external-roots-function
 backend implementation of `project-external-roots'.")
 
 (defun project-try-vc (dir)
+  (defvar vc-svn-admin-directory)
+  (require 'vc-svn)
+  ;; FIXME: Learn to invalidate when the value of
+  ;; `project-vc-merge-submodules' or `project-vc-extra-root-markers'
+  ;; changes.
   (or (vc-file-getprop dir 'project-vc)
-      (let* ((backend (ignore-errors (vc-responsible-backend dir)))
+      (let* ((backend-markers-alist `((Git . ".git")
+                                      (Hg . ".hg")
+                                      (Bzr . ".bzr")
+                                      (SVN . ,vc-svn-admin-directory)
+                                      (DARCS . "_darcs")
+                                      (Fossil . ".fslckout")))
+             (backend-markers
+              (delete
+               nil
+               (mapcar
+                (lambda (b) (assoc-default b backend-markers-alist))
+                vc-handled-backends)))
+             (marker-re
+              (mapconcat
+               (lambda (m) (format "\\(%s\\)" (wildcard-to-regexp m)))
+               (append backend-markers project-vc-extra-root-markers)
+               "\\|"))
+             (locate-dominating-stop-dir-regexp
+              (or vc-ignore-dir-regexp locate-dominating-stop-dir-regexp))
+             last-matches
              (root
-              (pcase backend
-                ('Git
-                 ;; Don't stop at submodule boundary.
-                 (or (vc-file-getprop dir 'project-git-root)
-                     (let ((root (vc-call-backend backend 'root dir)))
-                       (vc-file-setprop
-                        dir 'project-git-root
-                        (if (and
-                             ;; FIXME: Invalidate the cache when the value
-                             ;; of this variable changes.
-                             project-vc-merge-submodules
-                             (project--submodule-p root))
-                            (let* ((parent (file-name-directory
-                                            (directory-file-name root))))
-                              (vc-call-backend backend 'root parent))
-                          root)))))
-                ('nil nil)
-                (_ (ignore-errors (vc-call-backend backend 'root dir)))))
+              (locate-dominating-file
+               dir
+               (lambda (d)
+                 (setq last-matches (directory-files d nil marker-re t 100)))))
+             (backend
+              (cl-find-if
+               (lambda (b)
+                 (member (assoc-default b backend-markers-alist)
+                         last-matches))
+               vc-handled-backends))
              project)
+        (when (and
+               (eq backend 'Git)
+               project-vc-merge-submodules
+               (project--submodule-p root))
+          (let* ((parent (file-name-directory (directory-file-name root))))
+            (setq root (vc-call-backend 'Git 'root parent))))
         (when root
+          (let* ((relative-dir (file-relative-name dir root))
+                 (subproject (seq-find
+                              (lambda (sub-dir)
+                                (string-prefix-p (file-name-as-directory sub-dir)
+                                                 relative-dir))
+                              project-vc-subprojects)))
+            (and subproject
+                 (setq root (concat root subproject))))
           (setq project (list 'vc backend root))
           ;; FIXME: Cache for a shorter time.
           (vc-file-setprop dir 'project-vc project)
@@ -626,7 +704,8 @@ project-ignores
   (let* ((root (nth 2 project))
          backend)
     (append
-     (when (file-equal-p dir root)
+     (when (and backend
+                (file-equal-p dir root))
        (setq backend (cadr project))
        (delq
         nil