Coordinates: 39°58′20″N 125°48′12″E / 39.97222°N 125.80333°E / 39.97222; 125.80333 (Unsan)

The Battle of Unsan (Korean: 운산군 전투), also known as the Battle of Yunshan (Chinese: 云山战斗; pinyin: Yún Shān Zhàn Dòu), was a series of engagements of the Korean War that took place from 25 October to 4 November 1950 near Unsan, North Pyongan province in present-day North Korea. As part of the Chinese First Phase Campaign, the People's Republic of China's People's Volunteer Army made repeated attacks against the Republic of Korea 1st Infantry Division near Unsan beginning on 25 October, in an attempt to take advancing United Nations forces by surprise. In an accidental first encounter with the United States military during the Korean War, the Chinese 39th Corps attacked the unprepared US 8th Cavalry Regiment in Unsan on 1 November, resulting in one of the most devastating US losses of the Korean War.

By October 1950, the United Nations (UN) forces had successfully broken out of the Pusan Perimeter in the extreme south of Korea and begun an aggressive northward advance towards the Sino-Korean border, chasing the now defeated army of North Korea. With the destruction of the Korean People's Army (KPA), the US 1st Cavalry Division entered Pyongyang on 19 October, while the South Korean troops were rushing towards the Yalu River in all directions. As part of the Thanksgiving Offensive to end the war, Major General Frank W. Milburn, commander of the US I Corps, ordered the Republic of Korea (ROK) 1st Infantry Division to secure the Sup’ung Dam on the Yalu River by advancing through Unsan.