diff --git a/lib/mint/http2.ex b/lib/mint/http2.ex
index 227d6822..9ad856e0 100644
--- a/lib/mint/http2.ex
+++ b/lib/mint/http2.ex
@@ -382,12 +382,19 @@ defmodule Mint.HTTP2 do
     * `{:invalid_status_header, value}` - when the `:status` pseudo-header of a response
       is not a three-digit status code. `value` is the received value.
 
-    * `{:invalid_header_name, name}` - when a response header name is invalid, for
-      example because it contains uppercase characters.
+    * `{:invalid_request_method, method}` - when the request method is not a valid
+      HTTP token. `method` is the given method.
 
-    * `{:invalid_header_value, name, value}` - when a response header value is invalid,
-      for example because it contains control characters. `name` is the name of the
-      header and `value` is the invalid value.
+    * `{:invalid_request_target, target}` - when the request target (the `:path`
+      or `:authority` pseudo-header) is empty or contains characters that are not allowed
+      in a header value. `target` is the given target.
+
+    * `{:invalid_header_name, name}` - when a request or response header name is invalid,
+      for example because it contains a space or control characters.
+
+    * `{:invalid_header_value, name, value}` - when a request or response header value is
+      invalid, for example because it contains control characters. `name` is the name of
+      the header and `value` is the invalid value.
 
     * `{:invalid_content_length_header, value}` - when the `content-length` header of a
       response is not a non-negative integer. `value` is the received value.
@@ -619,6 +626,7 @@ defmodule Mint.HTTP2 do
       |> add_default_headers(body)
       |> sort_pseudo_headers_to_front()
 
+    validate_request_headers!(conn, headers)
     {conn, stream_id, ref} = open_stream(conn, method)
     {conn, payload} = encode_request_payload(conn, stream_id, headers, body)
     conn = send!(conn, payload)
@@ -1434,6 +1442,7 @@ defmodule Mint.HTTP2 do
     end
 
     trailer_headers = Headers.to_raw(trailers, _case_sensitive = false)
+    validate_request_trailers!(conn, trailer_headers)
     enabled_flags = [:end_headers, :end_stream]
     {conn, payload} = encode_header_block(conn, stream_id, trailer_headers, enabled_flags)
     conn = put_in(conn.streams[stream_id].state, :half_closed_local)
@@ -1741,6 +1750,74 @@ defmodule Mint.HTTP2 do
     end)
   end
 
+  # RFC 9113 8.2.1 and 8.3: a request whose field names or values contain characters
+  # outside the allowed set is malformed, and a sender must not generate it. The
+  # headers are validated after lowercasing and adding the pseudo-headers, so the
+  # method, the request target and caller-provided pseudo-headers are covered too.
+  # HTTP/2 framing isn't confused by these bytes, but an intermediary that forwards
+  # the request over HTTP/1.1 could be.
+  defp validate_request_headers!(conn, headers) do
+    Enum.each(headers, fn
+      {":method", value} ->
+        unless valid_method?(value) do
+          throw({:mint, conn, wrap_error({:invalid_request_method, value})})
+        end
+
+      {pseudo_header, value} when pseudo_header in [":path", ":authority"] ->
+        unless valid_request_target?(value) do
+          throw({:mint, conn, wrap_error({:invalid_request_target, value})})
+        end
+
+      {":" <> name = pseudo_header, value} ->
+        unless valid_field_name?(name) do
+          throw({:mint, conn, wrap_error({:invalid_header_name, pseudo_header})})
+        end
+
+        unless valid_field_value?(value) do
+          throw({:mint, conn, wrap_error({:invalid_header_value, pseudo_header, value})})
+        end
+
+      {name, value} ->
+        validate_request_field!(conn, name, value)
+    end)
+  end
+
+  # RFC 9113 8.1: trailers must not include pseudo-header fields, and the field
+  # character rules apply to them as to any other field.
+  defp validate_request_trailers!(conn, trailers) do
+    Enum.each(trailers, fn {name, value} -> validate_request_field!(conn, name, value) end)
+  end
+
+  defp validate_request_field!(conn, name, value) do
+    cond do
+      not valid_field_name?(name) ->
+        throw({:mint, conn, wrap_error({:invalid_header_name, name})})
+
+      not valid_field_value?(value) ->
+        throw({:mint, conn, wrap_error({:invalid_header_value, name, value})})
+
+      true ->
+        :ok
+    end
+  end
+
+  # RFC 9110 9.1: method = token.
+  defp valid_method?(<<>>), do: false
+  defp valid_method?(method), do: token_chars?(method)
+
+  defp token_chars?(<<char, rest::binary>>)
+       when char in ?0..?9 or char in ?a..?z or char in ?A..?Z or char in ~c"!#$%&'*+-.^_`|~",
+       do: token_chars?(rest)
+
+  defp token_chars?(<<_char, _rest::binary>>), do: false
+  defp token_chars?(<<>>), do: true
+
+  # RFC 9113 8.3.1: :path must not be empty for http and https URIs, and :authority
+  # carries the target of a CONNECT request. Both are field values, so RFC 9113 8.2.1
+  # forbids NUL, CR and LF in them and leading or trailing whitespace.
+  defp valid_request_target?(<<>>), do: false
+  defp valid_request_target?(target), do: valid_field_value?(target)
+
   ## Frame handling
 
   defp maybe_concat_and_handle_new_data(conn, data) do
@@ -3153,12 +3230,20 @@ defmodule Mint.HTTP2 do
     "invalid :status pseudo-header in the response: #{inspect(value)}"
   end
 
+  def format_error({:invalid_request_method, method}) do
+    "invalid request method: #{inspect(method)}"
+  end
+
+  def format_error({:invalid_request_target, target}) do
+    "invalid request target: #{inspect(target)}"
+  end
+
   def format_error({:invalid_header_name, name}) do
-    "invalid header name in the response: #{inspect(name)}"
+    "invalid header name: #{inspect(name)}"
   end
 
   def format_error({:invalid_header_value, name, value}) do
-    "invalid value for header #{inspect(name)} in the response: #{inspect(value)}"
+    "invalid value for header #{inspect(name)}: #{inspect(value)}"
   end
 
   def format_error({:invalid_content_length_header, value}) do
diff --git a/test/mint/http2/conn_test.exs b/test/mint/http2/conn_test.exs
index 2a80d667..0cc4ae4c 100644
--- a/test/mint/http2/conn_test.exs
+++ b/test/mint/http2/conn_test.exs
@@ -1479,6 +1479,139 @@ defmodule Mint.HTTP2Test do
     end
   end
 
+  describe "request header validation" do
+    for method <- ["", "GET /admin", "GET\t", "G\r\nET", "GET\0"] do
+      test "an invalid method #{inspect(method)} returns an error", %{conn: conn} do
+        assert {:error, %HTTP2{} = conn, error} =
+                 HTTP2.request(conn, unquote(method), "/", [], nil)
+
+        assert_http2_error error, {:invalid_request_method, unquote(method)}
+        assert_request_was_not_sent(conn)
+      end
+
+      test "an invalid explicitly passed :method #{inspect(method)} returns an error", %{
+        conn: conn
+      } do
+        assert {:error, ^conn, error} =
+                 HTTP2.request(conn, "GET", "/", [{":method", unquote(method)}], nil)
+
+        assert_http2_error error, {:invalid_request_method, unquote(method)}
+        assert_request_was_not_sent(conn)
+      end
+    end
+
+    for target <- ["", "/p\r\nath", "/pa\nth", "/pa\0th", " /path", "/path "] do
+      test "an invalid request target #{inspect(target)} returns an error", %{conn: conn} do
+        assert {:error, %HTTP2{} = conn, error} =
+                 HTTP2.request(conn, "GET", unquote(target), [], nil)
+
+        assert_http2_error error, {:invalid_request_target, unquote(target)}
+        assert_request_was_not_sent(conn)
+      end
+    end
+
+    for target <- ["", "example.com:443\r\n", "example.com:443\0"] do
+      test "an invalid CONNECT target #{inspect(target)} returns an error", %{conn: conn} do
+        assert {:error, %HTTP2{} = conn, error} =
+                 HTTP2.request(conn, "CONNECT", unquote(target), [], :stream)
+
+        assert_http2_error error, {:invalid_request_target, unquote(target)}
+        assert_request_was_not_sent(conn)
+      end
+    end
+
+    for name <- [
+          "",
+          "x-upper case",
+          "x\rfoo",
+          "x\nfoo",
+          "x\0foo",
+          "x:foo",
+          "x\x7Ffoo",
+          "x\xC3\xA4"
+        ] do
+      test "an invalid header name #{inspect(name)} returns an error", %{conn: conn} do
+        assert {:error, %HTTP2{} = conn, error} =
+                 HTTP2.request(conn, "GET", "/", [{unquote(name), "bar"}], nil)
+
+        assert_http2_error error, {:invalid_header_name, unquote(name)}
+        assert_request_was_not_sent(conn)
+      end
+    end
+
+    for value <- ["evil\r\nx-injected: 1", "b\rar", "b\nar", "b\0ar", " bar", "bar ", "bar\t"] do
+      test "an invalid header value #{inspect(value)} returns an error", %{conn: conn} do
+        assert {:error, %HTTP2{} = conn, error} =
+                 HTTP2.request(conn, "GET", "/", [{"foo", unquote(value)}], nil)
+
+        assert_http2_error error, {:invalid_header_value, "foo", unquote(value)}
+        assert_request_was_not_sent(conn)
+      end
+    end
+
+    test "an invalid value in an explicitly passed pseudo-header returns an error",
+         %{conn: conn} do
+      headers = [{":scheme", "https"}, {":path", "/ws"}, {":protocol", "web\r\nsocket"}]
+
+      assert {:error, %HTTP2{} = conn, error} =
+               HTTP2.request(conn, "CONNECT", "/ws", headers, :stream)
+
+      assert_http2_error error, {:invalid_header_value, ":protocol", "web\r\nsocket"}
+      assert_request_was_not_sent(conn)
+    end
+
+    test "an invalid explicitly passed :path pseudo-header returns an error", %{conn: conn} do
+      headers = [{":scheme", "https"}, {":path", "/w\r\ns"}, {":protocol", "websocket"}]
+
+      assert {:error, %HTTP2{} = conn, error} =
+               HTTP2.request(conn, "CONNECT", "/ws", headers, :stream)
+
+      assert_http2_error error, {:invalid_request_target, "/w\r\ns"}
+      assert_request_was_not_sent(conn)
+    end
+
+    test "header values may be empty or contain obs-text and inner whitespace", %{conn: conn} do
+      headers = [{"empty", ""}, {"foo", "b\xC3\xA4r \tbaz"}, {"A-b_c.d!", "v"}]
+
+      assert {:ok, %HTTP2{} = conn, _ref} = HTTP2.request(conn, "GET", "/", headers, nil)
+
+      assert_recv_frames [headers(hbf: hbf)]
+
+      assert [
+               {":method", "GET"},
+               {":path", "/"},
+               {":scheme", "https"},
+               {":authority", _},
+               {"user-agent", _},
+               {"empty", ""},
+               {"foo", "b\xC3\xA4r \tbaz"},
+               {"a-b_c.d!", "v"}
+             ] = server_decode_headers(hbf)
+
+      assert HTTP2.open?(conn)
+    end
+
+    for {name, value, reason} <- [
+          {"x\r\nfoo", "bar", {:invalid_header_name, "x\r\nfoo"}},
+          {":status", "200", {:invalid_header_name, ":status"}},
+          {"foo", "b\r\nar", {:invalid_header_value, "foo", "b\r\nar"}}
+        ] do
+      test "an invalid trailer #{inspect({name, value})} returns an error", %{conn: conn} do
+        {conn, ref} = open_request(conn, :stream)
+        assert_recv_frames [headers(stream_id: _stream_id)]
+
+        assert {:error, %HTTP2{} = conn, error} =
+                 HTTP2.stream_request_body(conn, ref, {:eof, [{unquote(name), unquote(value)}]})
+
+        assert_http2_error error, unquote(Macro.escape(reason))
+
+        refute_receive {:ssl, _socket, _data}, 100
+        assert HTTP2.open_request_count(conn) == 1
+        assert HTTP2.open?(conn)
+      end
+    end
+  end
+
   describe "response header validation" do
     for status <- ["abc", "", "+200", "2000", "20", "200 ", " 200", "1ab", "000", "099"] do
       test "an invalid :status of #{inspect(status)} is a stream error", %{conn: conn} do
@@ -4782,4 +4915,11 @@ defmodule Mint.HTTP2Test do
     assert is_reference(ref)
     {conn, ref}
   end
+
+  # A rejected request must not reach the server or take up a stream.
+  defp assert_request_was_not_sent(conn) do
+    refute_receive {:ssl, _socket, _data}, 100
+    assert HTTP2.open_request_count(conn) == 0
+    assert HTTP2.open?(conn)
+  end
 end