diff --git a/runtests.py b/runtests.py
index 3a1c74852..574da01b0 100755
--- a/runtests.py
+++ b/runtests.py
@@ -191,35 +191,31 @@ def prep_scratch(scratchdir, srcdir, tooldir, setfacl_nodef):
 
 
 def _is_test_path(path):
-    base = os.path.basename(path)
-    return base.endswith('.test') or base.endswith(_PY_TEST_SUFFIX)
+    return os.path.basename(path).endswith(_PY_TEST_SUFFIX)
 
 
 def _testbase(path):
     """Strip the test extension to get the canonical test name."""
     base = os.path.basename(path)
-    if base.endswith('.test'):
-        return base[:-len('.test')]
     if base.endswith(_PY_TEST_SUFFIX):
         return base[:-len(_PY_TEST_SUFFIX)]
     return base
 
 
 def collect_tests(suitedir, patterns):
-    """Collect test scripts (.test or _test.py) matching the given patterns."""
+    """Collect test scripts (_test.py) matching the given patterns."""
     if not patterns:
-        candidates = (glob.glob(os.path.join(suitedir, '*.test'))
-                      + glob.glob(os.path.join(suitedir, '*' + _PY_TEST_SUFFIX)))
+        candidates = glob.glob(os.path.join(suitedir, '*' + _PY_TEST_SUFFIX))
         tests = sorted(p for p in candidates if _is_test_path(p))
     else:
         seen = set()
         tests = []
         for pat in patterns:
             # Accept either bare name ("mkpath"), explicit extension, or glob.
-            if pat.endswith('.test') or pat.endswith('.py'):
+            if pat.endswith('.py'):
                 pats = [pat]
             else:
-                pats = [pat + '.test', pat + _PY_TEST_SUFFIX]
+                pats = [pat + _PY_TEST_SUFFIX]
             for p in pats:
                 for m in sorted(glob.glob(os.path.join(suitedir, p))):
                     if _is_test_path(m) and m not in seen:
diff --git a/testsuite/fleettest.py b/testsuite/fleettest.py
index a4f3f2ab8..b200604a1 100755
--- a/testsuite/fleettest.py
+++ b/testsuite/fleettest.py
@@ -83,7 +83,11 @@
 # source tree these point at, so it must be run from inside an rsync checkout
 # or given --repo PATH.
 REPO = Path.cwd()
-WORKFLOWS = REPO / ".github" / "workflows"
+# Source tree providing the test suite (runtests.py + testsuite/). Defaults to
+# REPO; --testsuite-repo decouples it so one tree is built and another's suite is
+# run against the result.
+TESTSUITE_REPO = REPO
+WORKFLOWS = TESTSUITE_REPO / ".github" / "workflows"
 
 # Fleet config (overridable with --fleet): ~/.fleettest.json is tried first, then
 # fleettest.json next to this script. The example template sits next to the
@@ -815,18 +819,35 @@ def main() -> int:
                     help="report per-target wall-clock (push/build/test) to find "
                     "the slowest target")
     ap.add_argument("--repo", help="rsync source tree to build (default: cwd)")
+    ap.add_argument("--testsuite-repo",
+                    help="rsync tree to take runtests.py + testsuite/ from "
+                    "(default: --repo). Build one tree and run another's test "
+                    "suite against it, e.g. --repo ../rsync-v3.4 --testsuite-repo .")
     ap.add_argument("--fleet", help="fleet config JSON (default: ~/.fleettest.json, "
                     "else fleettest.json next to this script)")
     ap.add_argument("--list", action="store_true", help="list targets and exit")
     args = ap.parse_args()
 
-    global REPO, WORKFLOWS
+    global REPO, WORKFLOWS, TESTSUITE_REPO
     REPO = Path(args.repo).resolve() if args.repo else Path.cwd()
-    WORKFLOWS = REPO / ".github" / "workflows"
-    if not args.cleanup and not (REPO / "runtests.py").is_file():
-        print(f"{REPO} is not an rsync source tree (no runtests.py); "
-              f"run from inside a checkout or pass --repo", file=sys.stderr)
-        return 2
+    TESTSUITE_REPO = Path(args.testsuite_repo).resolve() if args.testsuite_repo else REPO
+    # The expected-skip lists travel with the suite, so read workflows from the
+    # tree that provides the tests.
+    WORKFLOWS = TESTSUITE_REPO / ".github" / "workflows"
+    if not args.cleanup:
+        # The Python test suite (runtests.py + testsuite/) comes from
+        # TESTSUITE_REPO, so that is where runtests.py must live.  The build tree
+        # (REPO) only has to be a buildable rsync source -- it may be an older
+        # release whose runtests.py predates the Python suite, or lacks it.
+        if not (TESTSUITE_REPO / "runtests.py").is_file():
+            print(f"{TESTSUITE_REPO} has no runtests.py; run from inside a "
+                  f"checkout or pass --testsuite-repo a tree with the Python "
+                  f"test suite", file=sys.stderr)
+            return 2
+        if not (REPO / "rsync.h").is_file():
+            print(f"{REPO} is not an rsync source tree (no rsync.h); "
+                  f"run from inside a checkout or pass --repo", file=sys.stderr)
+            return 2
 
     if args.fleet:
         config_path = Path(args.fleet).resolve()
@@ -905,6 +926,19 @@ def main() -> int:
             print(f"git archive failed: {ar.stderr}", file=sys.stderr)
             return 2
 
+        # --testsuite-repo: overlay another tree's runtests.py + testsuite/ onto
+        # the built source (merge, no delete). Build REPO's rsync, but run
+        # TESTSUITE_REPO's suite against it. The leftover .test files from REPO
+        # are ignored by a Python runtests.py (it globs *_test.py).
+        if TESTSUITE_REPO != REPO:
+            ov = subprocess.run(
+                f"git -C {TESTSUITE_REPO} archive HEAD -- runtests.py testsuite "
+                f"| tar -x -C {staging}",
+                shell=True, capture_output=True, text=True)
+            if ov.returncode != 0:
+                print(f"testsuite overlay archive failed: {ov.stderr}", file=sys.stderr)
+                return 2
+
         # Tests that opt into the non-root pass (same for every target).
         args.nonroot_tests = discover_nonroot_tests(Path(staging) / "testsuite")