The Korean War began on 25 June 1950. On that day, "Union" was underway conducting landing exercises at Sagami Wan, Honshū, Japan. She stopped briefly at Yokosuka before arriving at Sasebo on 3 July for repairs. Repairs and training continued at Yokosuka until "Union" sailed to Yokohama on 11 July to embark Army troops and equipment for transportation to Pohang, Korea, on 18 July. Having delivered her cargo, the ship returned to Yokosuka on 25 July and conducted various exercises until 4 September when she arrived at Kobe, Japan, to reload. On 11 September, "Union" got underway for Jinsen, Korea, where boat landings took place four days later amidst mortar, machine gun, and rifle fire. On 21 September, "Union" departed for Sasebo, Japan, with seven casualties on board. After delivering the casualties, "Union" travelled to Kobe, Japan, arriving on 4 October. She set course for Inchon, Korea, that day and arrived four days later to unload marines and equipment. She stopped at Yonghung Man Kosen, Korea, for five days before arriving at Yokosuka on 2 November. "Union" then got underway for San Diego, California, returning to her home port on 22 November 1950.