diff --git a/tests/test_timer.py b/tests/test_timer.py
index 33de062..e66adc2 100644
--- a/tests/test_timer.py
+++ b/tests/test_timer.py
@@ -78,3 +78,13 @@ def test_timer_finished():
     sp._freeze("")
 
     assert re.search(r"\(\d+:\d{2}:\d{2}.\d{2}\)", sp._last_frame) is not None
+
+
+def test_timer_custom_format():
+    sp = yaspin(timer="#{}---{}#")
+
+    assert sp.elapsed_time == 0
+
+    sp._freeze("")
+
+    assert "#0:00:00---0#" in sp._last_frame
diff --git a/yaspin/api.py b/yaspin/api.py
index 07a847e..b1f2fb6 100644
--- a/yaspin/api.py
+++ b/yaspin/api.py
@@ -30,7 +30,7 @@ def yaspin(*args: Any, **kwargs: Any) -> Yaspin:
             of the text string.
         sigmap (dict, optional): Maps POSIX signals to their respective
             handlers.
-        timer (bool, optional): Prints a timer showing the elapsed time.
+        timer (bool | str, optional): Prints a timer showing the elapsed time.
 
     Returns:
         core.Yaspin: instance of the Yaspin class.
diff --git a/yaspin/core.py b/yaspin/core.py
index 798b5be..f175093 100644
--- a/yaspin/core.py
+++ b/yaspin/core.py
@@ -107,7 +107,7 @@ def __init__(  # pylint: disable=too-many-arguments
         reversal: bool = False,
         side: str = "left",
         sigmap: Optional[dict[signal.Signals, SignalHandlers]] = None,
-        timer: bool = False,
+        timer: Union[bool, str] = False,
     ) -> None:
         # Spinner
         self._spinner = self._set_spinner(spinner)
@@ -444,9 +444,11 @@ def _compose_out(self, frame: str, mode: Optional[str] = None) -> str:
         if self._side == "right":
             frame, text = text, frame
         # Timer
-        if self._timer:
+        if self._timer is not False:
+            # TODO the format string should allow showing custom groupings (such as total seconds or HH:MM:SS)
+            timer_format = self._timer if isinstance(self._timer, str) else " ({}.{:02.0f})"
             sec, fsec = divmod(round(100 * self.elapsed_time), 100)
-            text += " ({}.{:02.0f})".format(  # pylint: disable=consider-using-f-string
+            text += timer_format.format(  # pylint: disable=consider-using-f-string
                 timedelta(seconds=sec), fsec
             )
         # Mode