The Battle of the Pusan Perimeter, known in Korean as the Battle of the Naktong River Defense Line, was a large-scale battle between United Nations Command (UN) and North Korean forces lasting from August 4th to September 18th, 1950. It was one of the first major engagements of the Korean War. An army of 140,000 UN troops, having been pushed south to the brink of defeat, were rallied to make a final stand against the invading Korean People's Army (KPA), 98,000 men strong.

UN forces, having been repeatedly defeated by the advancing KPA, were forced back to the "Pusan Perimeter", a 140-mile (230 km) defense line around an area on the southeastern tip of South Korea that included the port of Busan (then spelt Pusan). The UN troops, consisting mostly of forces from the Republic of Korea Army (ROKA), United States, and United Kingdom, mounted a last stand around the perimeter, fighting off repeated KPA attacks for six weeks as they were engaged around the cities of Taegu, Masan, and Pohang and the Naktong River. The massive KPA assaults were unsuccessful in forcing the UN troops back farther from the perimeter, despite two major pushes in August and September.

North Korean troops, hampered by supply shortages and massive losses, continually staged attacks on UN forces in an attempt to penetrate the perimeter and collapse the line. The UN forces, however, used the port to amass an overwhelming advantage in troops, equipment, and logistics, and its navy and air forces remained unchallenged by the KPA during the fight. After six weeks, the KPA force collapsed and retreated in defeat after the UN force launched a counterattack at Inchon on September 15th, and the UN forces in the perimeter broke out the following day. The battle was the farthest the KPA would advance in the war, as subsequent fighting ground the war into a stalemate.