diff --git a/CHANGELOG.md b/CHANGELOG.md
index 85d3bcec57..e2f2e240c3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ### Fixed
 
+* Stricter type checking on values in `data=...`, ensure only primitive values that neatly map to `str` are allowed. (#3128)
 * Fix `app` type signature in `ASGITransport`. (#3109)
 
 ## 0.27.0 (21st February, 2024)
diff --git a/httpx/_content.py b/httpx/_content.py
index 786699f38f..bd45e3e966 100644
--- a/httpx/_content.py
+++ b/httpx/_content.py
@@ -10,6 +10,7 @@
     Iterable,
     Iterator,
     Mapping,
+    Tuple,
 )
 from urllib.parse import urlencode
 
@@ -17,13 +18,14 @@
 from ._multipart import MultipartStream
 from ._types import (
     AsyncByteStream,
+    PrimitiveData,
     RequestContent,
     RequestData,
     RequestFiles,
     ResponseContent,
     SyncByteStream,
 )
-from ._utils import peek_filelike_length, primitive_value_to_str
+from ._utils import peek_filelike_length
 
 __all__ = ["ByteStream"]
 
@@ -133,15 +135,36 @@ def encode_content(
     raise TypeError(f"Unexpected type for 'content', {type(content)!r}")
 
 
+def _coerce_type(key: str, value: PrimitiveData) -> Tuple[str, str]:
+    if not isinstance(key, str):
+        raise TypeError(
+            f"Request data keys must be str. " f"Got type {type(key).__name__!r}."
+        )
+
+    if value is True:
+        return (key, "true")
+    elif value is False:
+        return (key, "false")
+    elif value is None:
+        return (key, "")
+    elif isinstance(value, (str, int, float)):
+        return (key, str(value))
+
+    raise TypeError(
+        f"Request data values must be str, int, float, bool, or None. "
+        f"Got type {type(value).__name__!r} for key {key!r}."
+    )
+
+
 def encode_urlencoded_data(
     data: RequestData,
 ) -> tuple[dict[str, str], ByteStream]:
     plain_data = []
     for key, value in data.items():
         if isinstance(value, (list, tuple)):
-            plain_data.extend([(key, primitive_value_to_str(item)) for item in value])
+            plain_data.extend([_coerce_type(key, each) for each in value])
         else:
-            plain_data.append((key, primitive_value_to_str(value)))
+            plain_data.append(_coerce_type(key, value))
     body = urlencode(plain_data, doseq=True).encode("utf-8")
     content_length = str(len(body))
     content_type = "application/x-www-form-urlencoded"
diff --git a/tests/test_content.py b/tests/test_content.py
index 21c92dd799..9c3a72b376 100644
--- a/tests/test_content.py
+++ b/tests/test_content.py
@@ -200,7 +200,7 @@ async def test_urlencoded_content():
 
 @pytest.mark.anyio
 async def test_urlencoded_boolean():
-    request = httpx.Request(method, url, data={"example": True})
+    request = httpx.Request(method, url, data={"true": True, "false": False})
     assert isinstance(request.stream, typing.Iterable)
     assert isinstance(request.stream, typing.AsyncIterable)
 
@@ -209,11 +209,11 @@ async def test_urlencoded_boolean():
 
     assert request.headers == {
         "Host": "www.example.com",
-        "Content-Length": "12",
+        "Content-Length": "21",
         "Content-Type": "application/x-www-form-urlencoded",
     }
-    assert sync_content == b"example=true"
-    assert async_content == b"example=true"
+    assert sync_content == b"true=true&false=false"
+    assert async_content == b"true=true&false=false"
 
 
 @pytest.mark.anyio
@@ -252,6 +252,21 @@ async def test_urlencoded_list():
     assert async_content == b"example=a&example=1&example=true"
 
 
+def test_urlencoded_invalid_key():
+    with pytest.raises(TypeError) as e:
+        httpx.Request(method, url, data={123: "value"})  # type: ignore
+    assert str(e.value) == "Request data keys must be str. Got type 'int'."
+
+
+def test_urlencoded_invalid_value():
+    with pytest.raises(TypeError) as e:
+        httpx.Request(method, url, data={"key": {"this": "ain't json, buddy"}})
+    assert str(e.value) == (
+        "Request data values must be str, int, float, bool, or None. "
+        "Got type 'dict' for key 'key'."
+    )
+
+
 @pytest.mark.anyio
 async def test_multipart_files_content():
     files = {"file": io.BytesIO(b"<file content>")}