The Battle of Timor occurred in Portuguese Timor and Dutch Timor during the Second World War. Japanese forces invaded the island on 19 February 1942 and were resisted by a small, under-equipped force of Allied military personnel—known as Sparrow Force—predominantly from Australia, the United Kingdom, and the Dutch East Indies. Following a brief but stout resistance, the Japanese succeeded in forcing the surrender of the bulk of the Allied force after three days of fighting, although several hundred Australian commandos continued to wage an unconventional raiding campaign. They were resupplied by aircraft and vessels, based mostly in Darwin, Australia, about 650 km (400 mi) to the southeast, across the Timor Sea. During the subsequent fighting, the Japanese suffered heavy casualties, but they were eventually able to contain the Australians.

The campaign lasted until 10 February 1943, when the final remaining Australians were evacuated, making them the last Allied land forces to leave Southeast Asia following the Japanese offensives of 1941–1942. As a result, an entire Japanese division was tied up on Timor for more than six months, preventing its deployment elsewhere. Although Portugal was not a combatant, many Timorese and European Portuguese civilians fought with the Allies or provided them with food, shelter and other assistance. Some Timorese continued a resistance campaign following the Australian withdrawal. For this, they paid a heavy price and tens of thousands of Timorese civilians died as a result of the Japanese occupation, which lasted until the end of the war in 1945.

By late 1941, the island of Timor was divided politically between two colonial powers: the Portuguese in the east with a capital at Dili, and the Dutch in the west with an administrative centre at Kupang. A Portuguese exclave at Ocussi was also within the Dutch area. The Dutch defence included a force of 500 troops centred on Kupang, while the Portuguese force at Dili numbered just 150. In February, the Australian and Dutch governments had agreed that in the event Japan entered the Second World War on the Axis side, Australia would provide aircraft and troops to reinforce Dutch Timor. Portugal maintained its neutrality. As such, following the Japanese attack on Pearl Harbor, a small Australian force—known as Sparrow Force—arrived at Kupang on 12 December 1941. Meanwhile, two similar forces, known as Gull Force and Lark Force, were sent by the Australians to reinforce Ambon and Rabaul.

Sparrow Force was initially commanded by Lieutenant Colonel William Leggatt, and included the 2/40th Battalion, a commando unit—the 2nd Independent Company—under Major Alexander Spence, and a battery of coastal artillery. There were in total around 1,400 men. The force reinforced Royal Netherlands East Indies Army troops under the command of Lieutenant Colonel Nico van Straten, including the Timor and Dependencies Garrison Battalion, a company from the VIII Infantry Battalion, a reserve infantry company, a machine-gun platoon from the XIII Infantry Battalion and an artillery battery. Air support consisted of 12 Lockheed Hudson light bombers of No. 2 Squadron, Royal Australian Air Force (RAAF). Sparrow Force was initially deployed around Kupang, and the strategic airfield of Penfui in the south-west corner of the island, although other units were based at Klapalima, Usapa Besar and Babau, while a supply base was also established further east at Champlong.

Up to this point, the Government of Portugal had declined to co-operate with the Allies, relying on its claim of neutrality and plans to send an 800-strong force from Mozambique to defend the territory against a hypothetical Japanese invasion. However, this refusal left the Allied flank severely exposed, and a 400-man combined Dutch-Australian force subsequently occupied Portuguese Timor on 17 December. In response, the Portuguese Prime Minister, Salazar, protested to the Allied governments, while the governor of Portuguese Timor declared himself a prisoner in order to preserve the appearance of neutrality. Most of the Dutch troops and the whole of the 2/2nd Independent Company were subsequently transferred to Portuguese Timor and distributed in small detachments around the territory.

Neutral Portuguese Timor had not been originally included among the Japanese war objectives, but after Allied occupation violated its neutrality the Japanese decided to invade.

The Portuguese and the British governments reached an agreement that established the withdrawal of the Allied forces from Portuguese Timor, in exchange for the sending, by Portugal, of a military force to replace them. On 28 January 1942, the Portuguese force sailed from Lourenço Marques, Mozambique, heading for Timor, but the Japanese invasion occurred before it could arrive.

In January 1942, the Allied forces on Timor became a key link in the so-called "Malay Barrier", defended by the short-lived American-British-Dutch-Australian Command under the overall command of General Sir Archibald Wavell. Additional Australian support staff arrived at Kupang on 12 February, including Brigadier William Veale, who had been made the Allied commanding officer on Timor. By this time, many members of Sparrow Force—most of whom were unused to tropical conditions—were suffering from malaria and other illnesses. The airfield at Penfui in Dutch Timor also became a key air link between Australia and American forces fighting in the Philippines under General Douglas MacArthur. Penfui came under attack from Japanese aircraft on 26 and 30 January 1942, however the raids were hampered by the British anti-aircraft gunners and, to a lesser degree, by P-40 fighters of the 33rd Pursuit Squadron, United States Army Air Forces, 11 of which were based in Darwin. Later, another 500 Dutch troops and the British 79th Light Anti-Aircraft Battery arrived to reinforce Timor, while an additional Australian-American force was scheduled to arrive in February.