diff --git a/.strict-typing b/.strict-typing
index c9c1ab56377..af4bd4a9cf4 100644
--- a/.strict-typing
+++ b/.strict-typing
@@ -407,6 +407,7 @@ homeassistant.components.tile.*
 homeassistant.components.tilt_ble.*
 homeassistant.components.time.*
 homeassistant.components.time_date.*
+homeassistant.components.timer.*
 homeassistant.components.tod.*
 homeassistant.components.todo.*
 homeassistant.components.tolo.*
diff --git a/homeassistant/components/timer/__init__.py b/homeassistant/components/timer/__init__.py
index 17712b6aef1..4c611962436 100644
--- a/homeassistant/components/timer/__init__.py
+++ b/homeassistant/components/timer/__init__.py
@@ -4,7 +4,7 @@ from __future__ import annotations
 from collections.abc import Callable
 from datetime import datetime, timedelta
 import logging
-from typing import Self
+from typing import Any, Self, TypeVar
 
 import voluptuous as vol
 
@@ -28,6 +28,7 @@ from homeassistant.helpers.storage import Store
 from homeassistant.helpers.typing import ConfigType
 import homeassistant.util.dt as dt_util
 
+_T = TypeVar("_T")
 _LOGGER = logging.getLogger(__name__)
 
 DOMAIN = "timer"
@@ -73,14 +74,14 @@ STORAGE_FIELDS = {
 }
 
 
-def _format_timedelta(delta: timedelta):
+def _format_timedelta(delta: timedelta) -> str:
     total_seconds = delta.total_seconds()
     hours, remainder = divmod(total_seconds, 3600)
     minutes, seconds = divmod(remainder, 60)
     return f"{int(hours)}:{int(minutes):02}:{int(seconds):02}"
 
 
-def _none_to_empty_dict(value):
+def _none_to_empty_dict(value: _T | None) -> _T | dict[Any, Any]:
     if value is None:
         return {}
     return value
@@ -185,7 +186,7 @@ class TimerStorageCollection(collection.DictStorageCollection):
     @callback
     def _get_suggested_id(self, info: dict) -> str:
         """Suggest an ID based on the config."""
-        return info[CONF_NAME]
+        return info[CONF_NAME]  # type: ignore[no-any-return]
 
     async def _update_data(self, item: dict, update_data: dict) -> dict:
         """Return a new updated data object."""
@@ -193,7 +194,7 @@ class TimerStorageCollection(collection.DictStorageCollection):
         # make duration JSON serializeable
         if CONF_DURATION in update_data:
             data[CONF_DURATION] = _format_timedelta(data[CONF_DURATION])
-        return data
+        return data  # type: ignore[no-any-return]
 
 
 class Timer(collection.CollectionEntity, RestoreEntity):
@@ -231,24 +232,24 @@ class Timer(collection.CollectionEntity, RestoreEntity):
         return timer
 
     @property
-    def name(self):
+    def name(self) -> str | None:
         """Return name of the timer."""
         return self._config.get(CONF_NAME)
 
     @property
-    def icon(self):
+    def icon(self) -> str | None:
         """Return the icon to be used for this entity."""
         return self._config.get(CONF_ICON)
 
     @property
-    def state(self):
+    def state(self) -> str:
         """Return the current value of the timer."""
         return self._state
 
     @property
-    def extra_state_attributes(self):
+    def extra_state_attributes(self) -> dict[str, Any]:
         """Return the state attributes."""
-        attrs = {
+        attrs: dict[str, Any] = {
             ATTR_DURATION: _format_timedelta(self._running_duration),
             ATTR_EDITABLE: self.editable,
         }
@@ -264,9 +265,9 @@ class Timer(collection.CollectionEntity, RestoreEntity):
     @property
     def unique_id(self) -> str | None:
         """Return unique id for the entity."""
-        return self._config[CONF_ID]
+        return self._config[CONF_ID]  # type: ignore[no-any-return]
 
-    async def async_added_to_hass(self):
+    async def async_added_to_hass(self) -> None:
         """Call when entity is about to be added to Home Assistant."""
         # If we don't need to restore a previous state or no previous state exists,
         # start at idle
@@ -302,7 +303,7 @@ class Timer(collection.CollectionEntity, RestoreEntity):
             self.async_finish()
 
     @callback
-    def async_start(self, duration: timedelta | None = None):
+    def async_start(self, duration: timedelta | None = None) -> None:
         """Start a timer."""
         if self._listener:
             self._listener()
@@ -356,9 +357,9 @@ class Timer(collection.CollectionEntity, RestoreEntity):
         self.async_write_ha_state()
 
     @callback
-    def async_pause(self):
+    def async_pause(self) -> None:
         """Pause a timer."""
-        if self._listener is None:
+        if self._listener is None or self._end is None:
             return
 
         self._listener()
@@ -370,7 +371,7 @@ class Timer(collection.CollectionEntity, RestoreEntity):
         self.async_write_ha_state()
 
     @callback
-    def async_cancel(self):
+    def async_cancel(self) -> None:
         """Cancel a timer."""
         if self._listener:
             self._listener()
@@ -385,9 +386,9 @@ class Timer(collection.CollectionEntity, RestoreEntity):
         self.async_write_ha_state()
 
     @callback
-    def async_finish(self):
+    def async_finish(self) -> None:
         """Reset and updates the states, fire finished event."""
-        if self._state != STATUS_ACTIVE:
+        if self._state != STATUS_ACTIVE or self._end is None:
             return
 
         if self._listener:
@@ -405,9 +406,9 @@ class Timer(collection.CollectionEntity, RestoreEntity):
         self.async_write_ha_state()
 
     @callback
-    def _async_finished(self, time):
+    def _async_finished(self, time: datetime) -> None:
         """Reset and updates the states, fire finished event."""
-        if self._state != STATUS_ACTIVE:
+        if self._state != STATUS_ACTIVE or self._end is None:
             return
 
         self._listener = None
diff --git a/mypy.ini b/mypy.ini
index 42051b4ce82..ce0b4a3575c 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -3832,6 +3832,16 @@ disallow_untyped_defs = true
 warn_return_any = true
 warn_unreachable = true
 
+[mypy-homeassistant.components.timer.*]
+check_untyped_defs = true
+disallow_incomplete_defs = true
+disallow_subclassing_any = true
+disallow_untyped_calls = true
+disallow_untyped_decorators = true
+disallow_untyped_defs = true
+warn_return_any = true
+warn_unreachable = true
+
 [mypy-homeassistant.components.tod.*]
 check_untyped_defs = true
 disallow_incomplete_defs = true