The Battle of Rabaul, also known by the Japanese as Operation R, an instigating action of the New Guinea campaign, was fought on the island of New Britain in the Australian Territory of New Guinea, from 23 January into February 1942. It was a strategically significant defeat of Allied forces by Japan in the Pacific campaign of World War II, with the Japanese invasion force quickly overwhelming the small Australian garrison, the majority of which was either killed or captured. Hostilities on the neighbouring island of New Ireland are usually considered to be part of the same battle. Rabaul was significant because of its proximity to the Japanese territory of the Caroline Islands, site of a major Imperial Japanese Navy base on Truk.

Following the capture of the port of Rabaul, Japanese forces turned it into a major base and proceeded to land on mainland New Guinea, advancing toward Port Moresby. Heavy fighting followed along the Kokoda Track, and around Milne Bay, before the Japanese were eventually pushed back towards Buna–Gona by early 1943. As part of Operation Cartwheel, throughout 1943–1945, Allied forces later sought to isolate the Japanese garrison on Rabaul, rather than capturing it, largely using air power to do so, with US and Australian ground forces pursuing a limited campaign in western New Britain during this time.

By the end of the war, there was still a sizeable garrison at Rabaul, with large quantities of equipment that were subsequently abandoned. In the aftermath, it took the Allies over two years to repatriate the captured Japanese soldiers, while clean up efforts continued past the late 1950s. Many relics including ships, aircraft and weapons, as well as abandoned positions and tunnels, remain in the area.