Ishibashi Tanzan (石橋 湛山 Ishibashi Tanzan, also referred as Tanzan Ishibashi September 25,1884–April 25,1973) was a Japanese journalist and politician. He was the 55th Prime Minister of Japan from December 23,1956 to February 25,1957. During the same time he was the 2nd president of the Liberal Democratic Party which was the first party at the Diet.

He was born in Tokyo and graduated from Waseda University. He worked as a journalist at the "Mainichi Shimbun" for a while. After he finished military service, he joined "Toyo Keizai Shimpo" (Eastern Economic Journal) and later he became the president of it. He wrote many opinions about Japanese financial policy and had argument with Inoue Junnosuke. Ishibashi had a liberal political view and espoused the Small Japan policy; the core opinion of his Small Japan policy was the abandonment of Manchuria.

After World War II he received some political offers. The Japan Socialist Party offered to make him a candidate. Ishibashi became the minister of Finance under the first cabinet of Shigeru Yoshida from 1946 to 1947. In 1947 he was purged and therefore retired as both a politician and a journalist. After his purge was canceled in 1951, he allied with Ichiro Hatoyama and joined the movement against Yoshida's cabinet. In 1953 Ishibashi was appointed to the minister of Industry by Hatoyama who became the prime minister. In 1955 the Liberal Democratic Party was found and Ishibashi joined it.

When Hatoyama decided to retire in 1956, the LDP held a vote for their new president. First Nobusuke Kishi was considered as the most likely candidate, but Ishibashi allied with another candidate (Kojiro Ishii ), and Ishibashi won the election. Ishibashi was appointed to the president and became the prime minister of Japan. Ishibashi stated the government should endeavor to set up diplomatic relations with the People's Republic of China and his policy was popular among the people. Unfortunately he became sick and gave up his office only two months later.

After he resigned the post of prime minister and post of president of LDP, he visited China in 1956. He was known as a prominent person among liberal politicians in the LDP. He opposed Kishi's politics on security, which seems to Ishibashi to be too militant.