Tsuyoshi Hasegawa (長谷川 毅 Hasegawa Tsuyoshi , born 1941)  is a Japanese historian, who teaches at the University of California, Santa Barbara, where he is director of the Cold War Studies program. He received his Ph.D. from University of Washington in 1969.
Hasegawa's current field of research includes the political history of the Russian Revolution of 1917 and Soviet–Japanese relations. He also reads and speaks English, Japanese, and Russian, which gives him a different perspective when analyzing Soviet-Japanese-US relations.
In his 2005 book, Racing the Enemy, Hasegawa puts forward the view that the atomic bombings of Hiroshima and Nagasaki were not the main decisive factor in the Japanese decision to surrender, ending World War II, specifically the Pacific Theater. Instead, Hasegawa looks to the breaking of the Neutrality Pact by the Soviet Union, and the imminent fall of Manchuria and Korea to the Soviet invasion of Manchuria. This view is in contrast to earlier critics of the bombing, who argued that US President Harry S. Truman's underlying objective was showcasing US military might, as a deterrent to Soviet leader Joseph Stalin's ambitions. Hasegawa emphasizes the extent to which Japanese decision-making was independent of the nuclear attacks. According to British historian Geoffrey Jukes: "[Hasegawa] demonstrates conclusively that it was the Soviet declaration of war, not the atomic bombs, that forced the Japanese to surrender unconditionally."
- Racing the Enemy: Stalin, Truman, and the Surrender of Japan. The Belnap Press of Harvard University Press, 2006. ISBN 978-0-674-01693-4
- The Northern Territories Dispute and Russo-Japanese Relations. Vol. 1: Between War and Peace, 1967–1985. Vol. 2: Neither War Nor Peace, 1985–1998. (Berkeley: International and Area Studies Publications, University of California at Berkeley, 1998.)
- Edited with Jonathan Haslam and Andrew Kuchins, Russia and Japan: An Unresolved Dilemma between Distant Neighbors (UC Berkeley, International and Area Studies, 1993).
- Roshia kakumeika petorogurado no shiminseikatsu ["Everyday Life of Petrograd during the Russian Revolution"] (Chuokoronsha, 1989).
- The February Revolution of Petrograd, 1917 (U. Washington Press, 1981). Table of Contents, Precis.
- Tsuyoshi Hasegawa, Racing the Enemy, inside cover.
- "Tsuyoshi Hasegawa". UCSB, Department of History. Retrieved 2008-03-23.
- Dominick Jenkins (August 6, 2005). "The bomb didn't win it". The Guardian. Retrieved 2008-03-23.
- Jukes, Geoffrey (2008). "Review of Tsuyoshi Hasegawa, Racing the enemy: Stalin, Truman, and the surrender of Japan (2006)". Australian Slavonic and East European Studies (St. Lucia, QLD: School of Languages and Comparative Cultural Studies, The University of Queensland) 22 (1–2). ISSN 0818-8149.