diff --git a/lnt/tests/test_suite.py b/lnt/tests/test_suite.py
index 7c93e5c6..2a842369 100644
--- a/lnt/tests/test_suite.py
+++ b/lnt/tests/test_suite.py
@@ -285,23 +285,30 @@ def run_test(self, opts):
             opts.only_test = opts.single_result
 
         if opts.only_test:
-            if not opts.exec_mode:
-                # Only validate against test_suite_root if we're not in exec mode
-                # --only-test can either point to a particular test or a directory.
-                # Therefore, test_suite_root + opts.only_test or
-                # test_suite_root + dirname(opts.only_test) must be a directory.
-                path = os.path.join(opts.test_suite_root, opts.only_test)
-                parent_path = os.path.dirname(path)
-
-                if os.path.isdir(path):
+            # Convert --only-test to tuple format: (directory, test_name or None)
+            # In non-exec mode, validate against test_suite_root
+            # In exec mode, validate against build_dir if available
+            base_path = opts.build_dir if opts.exec_mode and opts.build_dir else opts.test_suite_root
+
+            if base_path:
+                full_path = os.path.join(base_path, opts.only_test)
+                parent_path = os.path.dirname(full_path)
+
+                if os.path.isdir(full_path):
                     opts.only_test = (opts.only_test, None)
                 elif os.path.isdir(parent_path):
                     opts.only_test = (os.path.dirname(opts.only_test),
                                       os.path.basename(opts.only_test))
-                else:
+                elif not opts.exec_mode:
+                    # In normal mode, path must exist
                     self._fatal("--only-test argument not understood (must be a " +
-                                " test or directory name)")
-            # else: in exec mode, we'll use only_test as-is for filtering
+                                "test or directory name)")
+                else:
+                    # In exec mode, path may not exist yet, assume it's a directory
+                    opts.only_test = (opts.only_test, None)
+            else:
+                # No base_path available (e.g., exec_interleaved_builds), assume directory
+                opts.only_test = (opts.only_test, None)
 
         if opts.single_result and not opts.only_test[1]:
             self._fatal("--single-result must be given a single test name, "
diff --git a/tests/runtest/test_suite-build-dir.shtest b/tests/runtest/test_suite-build-dir.shtest
index 4abd32bc..40075ac9 100644
--- a/tests/runtest/test_suite-build-dir.shtest
+++ b/tests/runtest/test_suite-build-dir.shtest
@@ -63,3 +63,16 @@
 # RUN:     > %t.err2.log 2> %t.err2.err
 # RUN: filecheck --check-prefix CHECK-ERR-UNCONFIGURED < %t.err2.err %s
 # CHECK-ERR-UNCONFIGURED: --build-dir is not a configured build
+
+# Test 5: --exec with --build-dir and --only-test (tuple conversion test)
+# RUN: lnt runtest test-suite \
+# RUN:     --sandbox %t.SANDBOX \
+# RUN:     --no-timestamp \
+# RUN:     --exec \
+# RUN:     --build-dir %t.CUSTOM_BUILD/mybuild \
+# RUN:     --only-test SingleSource \
+# RUN:     --use-cmake %S/Inputs/test-suite-cmake/fake-cmake \
+# RUN:     --use-lit %S/Inputs/test-suite-cmake/fake-lit \
+# RUN:     > %t.exec-only.log 2> %t.exec-only.err
+# RUN: filecheck --check-prefix CHECK-EXEC-ONLY < %t.exec-only.err %s
+# CHECK-EXEC-ONLY: Testing