diff --git a/httpx/_models.py b/httpx/_models.py
index 2cc86321a4..d4e2c4ed66 100644
--- a/httpx/_models.py
+++ b/httpx/_models.py
@@ -396,7 +396,7 @@ def __init__(
         extensions: RequestExtensions | None = None,
     ) -> None:
         self.method = method.upper()
-        self.url = URL(url) if params is None else URL(url, params=params)
+        self.url = URL(url) if params is None else URL(url).copy_merge_params(params)
         self.headers = Headers(headers)
         self.extensions = {} if extensions is None else dict(extensions)
 
diff --git a/tests/models/test_requests.py b/tests/models/test_requests.py
index b31fe007be..321b611b73 100644
--- a/tests/models/test_requests.py
+++ b/tests/models/test_requests.py
@@ -232,10 +232,18 @@ def test_request_params():
     request = httpx.Request("GET", "http://example.com", params={})
     assert str(request.url) == "http://example.com"
 
+    # Params are merged with existing URL query parameters, not replaced.
     request = httpx.Request(
         "GET", "http://example.com?c=3", params={"a": "1", "b": "2"}
     )
-    assert str(request.url) == "http://example.com?a=1&b=2"
+    assert str(request.url) == "http://example.com?c=3&a=1&b=2"
 
+    # Empty params preserves existing URL query parameters.
     request = httpx.Request("GET", "http://example.com?a=1", params={})
-    assert str(request.url) == "http://example.com"
+    assert str(request.url) == "http://example.com?a=1"
+
+    # Params with overlapping keys override existing URL query parameters.
+    request = httpx.Request(
+        "GET", "http://example.com?a=1&b=2", params={"b": "3", "c": "4"}
+    )
+    assert str(request.url) == "http://example.com?a=1&b=3&c=4"