diff --git a/httpx/_content.py b/httpx/_content.py
index 24a967d506..eb7a7aef17 100644
--- a/httpx/_content.py
+++ b/httpx/_content.py
@@ -150,7 +150,7 @@ def encode_urlencoded_data(
 
 
 def encode_multipart_data(
-    data: dict, files: RequestFiles, boundary: Optional[bytes] = None
+    data: dict, files: RequestFiles, boundary: Optional[bytes]
 ) -> Tuple[Dict[str, str], MultipartStream]:
     multipart = MultipartStream(data=data, files=files, boundary=boundary)
     headers = multipart.get_headers()
diff --git a/httpx/_models.py b/httpx/_models.py
index 8879532c81..fd1d7fe9a1 100644
--- a/httpx/_models.py
+++ b/httpx/_models.py
@@ -27,6 +27,7 @@
     StreamConsumed,
     request_context,
 )
+from ._multipart import get_multipart_boundary_from_content_type
 from ._status_codes import codes
 from ._types import (
     AsyncByteStream,
@@ -332,7 +333,18 @@ def __init__(
             Cookies(cookies).set_cookie_header(self)
 
         if stream is None:
-            headers, stream = encode_request(content, data, files, json)
+            content_type: typing.Optional[str] = self.headers.get("content-type")
+            headers, stream = encode_request(
+                content=content,
+                data=data,
+                files=files,
+                json=json,
+                boundary=get_multipart_boundary_from_content_type(
+                    content_type=content_type.encode(self.headers.encoding)
+                    if content_type
+                    else None
+                ),
+            )
             self._prepare(headers)
             self.stream = stream
             # Load the request body, except for streaming content.
diff --git a/httpx/_multipart.py b/httpx/_multipart.py
index d42f5cb31b..8bd7a17c9b 100644
--- a/httpx/_multipart.py
+++ b/httpx/_multipart.py
@@ -20,6 +20,20 @@
 )
 
 
+def get_multipart_boundary_from_content_type(
+    content_type: typing.Optional[bytes],
+) -> typing.Optional[bytes]:
+    if not content_type or not content_type.startswith(b"multipart/form-data"):
+        return None
+    # parse boundary according to
+    # https://www.rfc-editor.org/rfc/rfc2046#section-5.1.1
+    if b";" in content_type:
+        for section in content_type.split(b";"):
+            if section.strip().lower().startswith(b"boundary="):
+                return section.strip()[len(b"boundary=") :].strip(b'"')
+    return None
+
+
 class DataField:
     """
     A single form field item, within a multipart form field.
diff --git a/tests/test_multipart.py b/tests/test_multipart.py
index 46ad0e01a1..dc93d26505 100644
--- a/tests/test_multipart.py
+++ b/tests/test_multipart.py
@@ -42,6 +42,58 @@ def test_multipart(value, output):
     assert multipart["file"] == [b"<file content>"]
 
 
+@pytest.mark.parametrize(
+    "header",
+    [
+        "multipart/form-data; boundary=+++; charset=utf-8",
+        "multipart/form-data; charset=utf-8; boundary=+++",
+        "multipart/form-data; boundary=+++",
+        "multipart/form-data; boundary=+++ ;",
+        'multipart/form-data; boundary="+++"; charset=utf-8',
+        'multipart/form-data; charset=utf-8; boundary="+++"',
+        'multipart/form-data; boundary="+++"',
+        'multipart/form-data; boundary="+++" ;',
+    ],
+)
+def test_multipart_explicit_boundary(header: str) -> None:
+    client = httpx.Client(transport=httpx.MockTransport(echo_request_content))
+
+    files = {"file": io.BytesIO(b"<file content>")}
+    headers = {"content-type": header}
+    response = client.post("http://127.0.0.1:8000/", files=files, headers=headers)
+    assert response.status_code == 200
+
+    # We're using the cgi module to verify the behavior here, which is a
+    # bit grungy, but sufficient just for our testing purposes.
+    assert response.request.headers["Content-Type"] == header
+    content_length = response.request.headers["Content-Length"]
+    pdict: dict = {
+        "boundary": b"+++",
+        "CONTENT-LENGTH": content_length,
+    }
+    multipart = cgi.parse_multipart(io.BytesIO(response.content), pdict)
+
+    assert multipart["file"] == [b"<file content>"]
+
+
+@pytest.mark.parametrize(
+    "header",
+    [
+        "multipart/form-data; charset=utf-8",
+        "multipart/form-data; charset=utf-8; ",
+    ],
+)
+def test_multipart_header_without_boundary(header: str) -> None:
+    client = httpx.Client(transport=httpx.MockTransport(echo_request_content))
+
+    files = {"file": io.BytesIO(b"<file content>")}
+    headers = {"content-type": header}
+    response = client.post("http://127.0.0.1:8000/", files=files, headers=headers)
+
+    assert response.status_code == 200
+    assert response.request.headers["Content-Type"] == header
+
+
 @pytest.mark.parametrize(("key"), (b"abc", 1, 2.3, None))
 def test_multipart_invalid_key(key):
     client = httpx.Client(transport=httpx.MockTransport(echo_request_content))