Hamaguchi Osachi (Kyūjitai: 濱口 雄幸; Shinjitai: 浜口 雄幸, 1 April 1870 – 26 August 1931) was a Japanese politician who served as Prime Minister of Japan from 1929 to 1931. Nicknamed the "Lion Prime Minister" (ライオン宰相, Raion Saishō) due to his dignified demeanor and mane-like hair, Hamaguchi served as leading member of the liberal Rikken Minseitō (Constitutional Democratic Party) during the "Taishō Democracy" of interwar Japan. He was a member of the House of Representatives from 1915 until his death. He initially survived an assassination attempt by a right-wing extremist in 1930, but died about nine months later from a bacterial infection in his unhealed wounds.

Hamaguchi was born in Nagaoka District, Tosa Province (now part of Kōchi city, Kōchi Prefecture on the island of Shikoku). He was the third son of Minaguchi Tanehira, an official in the local forestry department, and took the Hamaguchi name on his marriage to Hamaguchi Natsuko in 1889.

Hamaguchi graduated from the Law College of Tokyo Imperial University in 1895 and began his career as a bureaucrat in the Ministry of Finance.

In 1907, he rose to the position of Director of the Monopoly Bureau. He became Vice Communications Minister in 1912 and Vice Finance Minister in 1914.

Hamaguchi joined the Rikken Dōshikai political party led by Katō Takaaki in 1915, which became the Kenseikai in 1916. Hamaguchi was elected to the lower house in the Japanese Diet in 1915 from the Kōchi Second District, and was to hold onto this seat until his death in 1931.

In June 1924, Hamaguchi served as Finance Minister under the Katō Takaaki Cabinet, holding the same portfolio under the First Wakatsuki Cabinet from January to June 1926. As Finance Minister, he pursued fiscal retrenchment, and proposed reducing government spending by 17 percent and laying off tens of thousands of government workers; however, his policies had to be scaled considerably back due to strenuous opposition from government bureaucrats.

Hamaguchi was subsequently Home Minister in the Wakatsuki cabinet from June 1926 to April 1927. In a continuation of his efforts while as Finance Minister, Hamaguchi promoted a moral campaign through sponsorship of movies which emphasized thrift and reduced public consumption, with the goal of helping reduce Japan's trade deficit.

In 1927, Hamaguchi became the chairman of the new Rikken Minseitō political party formed by the merger of the Kenseikai and the Seiyūhontō.