Rabaul (/rɑːˈbaʊl/) is a township in the East New Britain province of Papua New Guinea, on the island of New Britain. It lies about 600 km (370 mi) to the east of the island of New Guinea. Rabaul was the provincial capital and most important settlement in the province until it was destroyed in 1994 by falling ash from a volcanic eruption in its harbor. During the eruption, ash was sent thousands of metres into the air, and the subsequent rain of ash caused 80% of the buildings in Rabaul to collapse. After the eruption the capital was moved to Kokopo, about 20 kilometres (12 mi) away. Rabaul is continually threatened by volcanic activity, because it is on the edge of the Rabaul caldera, a flooded caldera of a large pyroclastic shield volcano.

Rabaul was planned and built around the harbour area known as Simpsonhafen (Simpson Harbour) during the German New Guinea administration, which controlled the region from 1884 and formally through 1919. Rabaul was selected as the capital of the German New Guinea administration in 1905, and the administrative offices were transferred there in 1910. Rabaul was captured by the British Empire during the early days of World War I. It became the capital of the Australian-mandated Territory of New Guinea until 1937, when it was first destroyed by a volcano. During World War II, it was captured by Japan in 1942 and became its main base of military and naval activity in the South Pacific. Settlements and military installations around the edge of the caldera are often collectively called Rabaul, although the old town of Rabaul was reduced to practical insignificance by the volcanic eruption in 1937.

As a tourist destination, Rabaul is popular for its volcanoes, scuba diving and for snorkelling sites, spectacular harbour and other scenery, World War II history, flora and fauna, and the cultural life of the Tolai people. Before the 1994 eruption, Rabaul was a popular commercial and recreational boating destination; fewer private small craft visit now, but 10 to 12 cruise ships visit Rabaul each year, including the Queen Elizabeth, carrying up to 2,000 passengers. Tourism is a major industry in Rabaul and East New Britain generally.

Rabaul's proximity to its volcanoes has always been a source of concern. It could be the source of the 1808 mystery eruption. In 1878, before it was established as a town, an eruption formed a volcano in the harbour.

For other eruptions, see Rabaul caldera.

In 1910 the German colonial government during the administration of Governor Albert Hahl moved offices, the district court, a hospital and customs and postal facilities from Herbertshöhe (today's Kokopo) to Simpsonhafen. That settlement was thus substantially enlarged with official buildings and housing and renamed Rabaul, meaning mangrove in Kuanua (the local language) as the new town was partially built on a reclaimed mangrove swamp.

At the outset of World War I, at the behest of Great Britain, Australia – as one of the Dominions of the British Empire – defeated the German military garrison in Rabaul and occupied the territory with the volunteer Australian Naval and Military Expeditionary Force. Following Germany's defeat at the end of the war, the occupied territory was delegated in 1920 to Australia as a League of Nations Mandate (Class C). Rabaul became the capital of the Territory of New Guinea. Visits to and stays in Rabaul during this period were amply described in books by many authors, including Margaret Mead.

Gunantambu, the famous house of "Queen" Emma Forsayth and her husband, contained furniture previously owned by Robert Louis Stevenson and left to her family in Samoa. Destroyed in the 1937 volcano eruption, its remains became a tourist attraction after World War II and remained so until the 1994 further volcanic destruction of Rabaul.