The start of the Korean War saw the reactivation of 3/1 on 4 August 1950 at Marine Corps Base Camp Pendleton, California and their quick deployment to Korea in September. The battalion's first action was at the Battle of Inchon in September 1950. Following the recapture of Seoul, 3/1, along with the rest of the 1st Marine Division, was put back on ship and sailed around to the east coast of Korea. They eventually landed at Wonsan in late October and from there participated in the Battle of Chosin Reservoir. During the battle they were the only battalion from the Chesty Puller's 1st Marine Regiment to make it as far north as Hagaru-ri. After the withdrawal from Chosin the 1st Marine Division was evacuated from Hungnam. The battalion took part in fighting on the East Central Front and Western Front of the Jamestown Line for the remainder of the war. After the Korean Armistice Agreement was signed, they participated in the defense of the Korean Demilitarized Zone from July 1953 to April 1955. In April 1955, the battalion re-located to MCB Camp Pendleton, California.