Following the outbreak of the Korean War, 1/1 was reactivated at MCB Camp Pendleton on August 9, 1950. Later that month they deployed to Kobe, Japan and from there took part in the amphibious landing during the Battle of Inchon. In October, the Marines were withdrawn from the Seoul area and moved to the east coast of Korea landing at Wosnan in late October. From there 1st Battalion 1st Marines participated in the Battle of Chosin Reservoir. They pushed as far north as Koto-ri, spending much of the battle defending their perimeter in this vicinity.
The battalion spent much of the remainder of the war defending the thirty-eighth parallel. All told, it fought in the Korean War from September 1950 through July 1953.
Following the war, the battalion participated in the defense of the Korean Demilitarized Zone from July 1953 to April 1955