Matsui Keishirō (松井 慶四郎?, March 28, 1868 – June 4, 1946) was a Japanese statesman and diplomat.
Matsui was a native of Osaka Prefecture, and a graduate of the Law School of Tokyo Imperial University in 1889. He entered the Ministry of Foreign Affairs the same year. In 1890, he was assigned to the Japanese embassy in Seoul, Korea, and in 1895 was assigned to the Japanese embassy in the United States. In 1898, he was promoted to the position of First Secretary at the Japanese Embassy in London, United Kingdom. In 1902, he was reassigned to the Japanese embassy in Beijing, China, returning to Japan in 1913.
During the First World War, served as Japanese Ambassador to France and was a plenipotentiary at the 1919 Paris Peace Conference. On the successful completion of this mission, he was awarded with the title of baron (danshaku) under the kazoku peerage system. He served as Japanese Minister of Foreign Affairs from January 7 to June 11, 1924 under the administration of Kiyoura Keigo and was also appointed a member of the House of Peers in the Diet of Japan. He later served as Ambassador to the United Kingdom in 1925–1928. In 1938, he became a member of the Privy Council.
- Phillips Payson O'Brien 2004. The Anglo-Japanese Alliance, 1902-1922, Routledge, ISBN 0-415-32611-7
- Ministry of Foreign Affairs
- Donald Hankey (3 June 2015). The Supreme Control at the Paris Peace Conference 1919 (Routledge Revivals): A Commentary. Routledge. pp. 43–. ISBN 978-1-317-56756-1.
- Obituary in the New York Times
|Minister for Foreign Affairs
January 1924 – June 1924