diff --git a/pydantic_ai_slim/pydantic_ai/models/gemini.py b/pydantic_ai_slim/pydantic_ai/models/gemini.py
index 64008622b..ee6a659b8 100644
--- a/pydantic_ai_slim/pydantic_ai/models/gemini.py
+++ b/pydantic_ai_slim/pydantic_ai/models/gemini.py
@@ -397,6 +397,95 @@ def _map_response_schema(self, o: OutputObjectDefinition) -> dict[str, Any]:
 
         return response_schema
 
+    async def count_tokens(
+        self,
+        messages: list[ModelMessage],
+        model_settings: GeminiModelSettings | None,
+        model_request_parameters: ModelRequestParameters,
+    ) -> usage.Usage:
+        check_allow_model_requests()
+        async with self._make_count_request(messages, model_settings or {}, model_request_parameters) as http_response:
+            data = await http_response.aread()
+            response = _gemini_count_tokens_response_ta.validate_json(data)
+        return self._process_count_tokens_response(response)
+
+    @asynccontextmanager
+    async def _make_count_request(
+        self,
+        messages: list[ModelMessage],
+        model_settings: GeminiModelSettings,
+        model_request_parameters: ModelRequestParameters,
+    ) -> AsyncIterator[HTTPResponse]:
+        tools = self._get_tools(model_request_parameters)
+        tool_config = self._get_tool_config(model_request_parameters, tools)
+        sys_prompt_parts, contents = await self._message_to_gemini_content(messages)
+
+        request_data = _GeminiCountTokensRequest(contents=contents)
+
+        generation_config = _settings_to_generation_config(model_settings)
+        if model_request_parameters.output_mode == 'native':
+            if tools:
+                raise UserError('Gemini does not support structured output and tools at the same time.')
+            generation_config['response_mime_type'] = 'application/json'
+            output_object = model_request_parameters.output_object
+            assert output_object is not None
+            generation_config['response_schema'] = self._map_response_schema(output_object)
+        elif model_request_parameters.output_mode == 'prompted' and not tools:
+            generation_config['response_mime_type'] = 'application/json'
+
+        if generation_config:
+            request_data['generateContentRequest'] = {
+                'contents': contents,
+                'generationConfig': generation_config,
+            }
+            if sys_prompt_parts:
+                request_data['generateContentRequest']['systemInstruction'] = _GeminiTextContent(
+                    role='user', parts=sys_prompt_parts
+                )
+            if tools is not None:
+                request_data['generateContentRequest']['tools'] = tools
+            if tool_config is not None:
+                request_data['generateContentRequest']['toolConfig'] = tool_config
+
+        headers = {'Content-Type': 'application/json', 'User-Agent': get_user_agent()}
+        url = f'/models/{self._model_name}:countTokens'
+
+        request_json = _gemini_count_tokens_request_ta.dump_json(request_data, by_alias=True)
+        async with self.client.stream(
+            'POST',
+            url,
+            content=request_json,
+            headers=headers,
+            timeout=model_settings.get('timeout', USE_CLIENT_DEFAULT),
+        ) as r:
+            if (status_code := r.status_code) != 200:
+                await r.aread()
+                if status_code >= 400:
+                    raise ModelHTTPError(status_code=status_code, model_name=self.model_name, body=r.text)
+                raise UnexpectedModelBehavior(  # pragma: no cover
+                    f'Unexpected response from gemini {status_code}', r.text
+                )
+            yield r
+
+    def _process_count_tokens_response(self, response: _GeminiCountTokensResponse) -> usage.Usage:
+        details: dict[str, int] = {}
+        if cached_content_token_count := response.get('cachedContentTokenCount'):
+            details['cached_content_tokens'] = cached_content_token_count
+
+        for key, metadata_details in response.items():
+            if key.endswith('TokensDetails') and metadata_details:
+                metadata_details = cast(list[_GeminiModalityTokenCount], metadata_details)
+                suffix = key.removesuffix('TokensDetails').lower()
+                for detail in metadata_details:
+                    details[f'{detail["modality"].lower()}_{suffix}'] = detail['token_count']
+
+        return usage.Usage(
+            request_tokens=response.get('totalTokens', 0),
+            response_tokens=0,  # countTokens does not provide response tokens
+            total_tokens=response.get('totalTokens', 0),
+            details=details,
+        )
+
 
 def _settings_to_generation_config(model_settings: GeminiModelSettings) -> _GeminiGenerationConfig:
     config: _GeminiGenerationConfig = {}
@@ -809,6 +898,34 @@ class _GeminiResponse(TypedDict):
     vendor_id: NotRequired[Annotated[str, pydantic.Field(alias='responseId')]]
 
 
+@pydantic.with_config(pydantic.ConfigDict(defer_build=True))
+class _GeminiCountTokensRequest(TypedDict):
+    """Schema for a countTokens API request to the Gemini API.
+
+    See <https://ai.google.dev/api/tokens#endpoint> for API docs.
+    """
+
+    contents: NotRequired[list[_GeminiContent]]
+    generateContentRequest: NotRequired[_GeminiRequest]
+
+
+@pydantic.with_config(pydantic.ConfigDict(defer_build=True))
+class _GeminiCountTokensResponse(TypedDict):
+    """Schema for the response from the Gemini countTokens API.
+
+    See <https://ai.google.dev/api/tokens#endpoint> for API docs.
+    """
+
+    totalTokens: int
+    cachedContentTokenCount: NotRequired[int]
+    promptTokensDetails: NotRequired[list[_GeminiModalityTokenCount]]
+    cacheTokensDetails: NotRequired[list[_GeminiModalityTokenCount]]
+
+
+_gemini_count_tokens_request_ta = pydantic.TypeAdapter(_GeminiCountTokensRequest)
+_gemini_count_tokens_response_ta = pydantic.TypeAdapter(_GeminiCountTokensResponse)
+
+
 class _GeminiCandidates(TypedDict):
     """See <https://ai.google.dev/api/generate-content#v1beta.Candidate>."""