diff --git a/haystack/tools/tool.py b/haystack/tools/tool.py
index 2fa314659..478f8a931 100644
--- a/haystack/tools/tool.py
+++ b/haystack/tools/tool.py
@@ -2,6 +2,7 @@
 #
 # SPDX-License-Identifier: Apache-2.0
 
+import inspect
 from dataclasses import asdict, dataclass
 from typing import Any, Callable, Optional
 
@@ -34,6 +35,7 @@ class Tool:
         A JSON schema defining the parameters expected by the Tool.
     :param function:
         The function that will be invoked when the Tool is called.
+        Must be a synchronous function; async functions are not supported.
     :param outputs_to_string:
         Optional dictionary defining how a tool outputs should be converted into a string.
         If the source is provided only the specified output key is sent to the handler.
@@ -74,6 +76,14 @@ class Tool:
     outputs_to_state: Optional[dict[str, dict[str, Any]]] = None
 
     def __post_init__(self):
+        # Check that the function is not a coroutine (async function)
+        if inspect.iscoroutinefunction(self.function):
+            raise ValueError(
+                f"Async functions are not supported as tools. "
+                f"The function '{self.function.__name__}' is a coroutine function. "
+                f"Please use a synchronous function instead."
+            )
+
         # Check that the parameters define a valid JSON schema
         try:
             Draft202012Validator.check_schema(self.parameters)
diff --git a/releasenotes/notes/raise-error-async-tool-56dc668744034656.yaml b/releasenotes/notes/raise-error-async-tool-56dc668744034656.yaml
new file mode 100644
index 000000000..908db506c
--- /dev/null
+++ b/releasenotes/notes/raise-error-async-tool-56dc668744034656.yaml
@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Raise a ``ValueError`` when an async function is passed to the ``Tool`` class. Async functions are not supported
+    as tools. This change provides a clear error message instead of silent failures where coroutines are never awaited.
diff --git a/test/tools/test_from_function.py b/test/tools/test_from_function.py
index 3ad2c94a2..ccedd329c 100644
--- a/test/tools/test_from_function.py
+++ b/test/tools/test_from_function.py
@@ -286,3 +286,21 @@ def test_remove_title_from_schema_handle_no_title_in_top_level():
         "properties": {"parameter1": {"type": "string"}, "parameter2": {"type": "integer"}},
         "type": "object",
     }
+
+
+def test_from_function_async_raises_error():
+    async def async_get_weather(city: str) -> str:
+        """Get weather report for a city."""
+        return f"Weather report for {city}: 20°C, sunny"
+
+    with pytest.raises(ValueError, match="Async functions are not supported as tools"):
+        create_tool_from_function(async_get_weather)
+
+
+def test_tool_decorator_async_raises_error():
+    with pytest.raises(ValueError, match="Async functions are not supported as tools"):
+
+        @tool
+        async def async_get_weather(city: str) -> str:
+            """Get weather report for a city."""
+            return f"Weather report for {city}: 20°C, sunny"
diff --git a/test/tools/test_tool.py b/test/tools/test_tool.py
index bc8c85cb0..0725274fc 100644
--- a/test/tools/test_tool.py
+++ b/test/tools/test_tool.py
@@ -21,6 +21,10 @@ def format_string(text: str) -> str:
 parameters = {"type": "object", "properties": {"city": {"type": "string"}}, "required": ["city"]}
 
 
+async def async_get_weather(city: str) -> str:
+    return f"Weather report for {city}: 20°C, sunny"
+
+
 class TestTool:
     def test_init(self):
         tool = Tool(
@@ -39,6 +43,10 @@ class TestTool:
         with pytest.raises(ValueError):
             Tool(name="irrelevant", description="irrelevant", parameters=params, function=get_weather_report)
 
+    def test_init_async_function_raises_error(self):
+        with pytest.raises(ValueError, match="Async functions are not supported as tools"):
+            Tool(name="weather", description="Get weather report", parameters=parameters, function=async_get_weather)
+
     @pytest.mark.parametrize(
         "outputs_to_state",
         [