The Cocos Islands mutiny was a failed mutiny by Sri Lankan soldiers against British officers, on the Cocos (Keeling) Islands on 8 May 1942, during the Second World War.

The mutineers attempted to seize control of the islands and disable the British garrison. However, the mutiny was defeated, the mutineers punished, and the three ringleaders executed; the only British Commonwealth servicemen to be executed for mutiny during the Second World War.

Units belonging to the Ceylon Defence Force (CDF), including the Ceylon Garrison Artillery (CGA), the Ceylon Light Infantry (CLI) and the Ceylon Volunteer Medical Corps, were mobilised on 2 September 1939, the day before Britain declared war on Nazi Germany. The CGA was equipped with six-inch (152 mm) and nine-inch (227 mm) guns. Several of them were posted to the Seychelles and the Cocos Islands, accompanied by contingents of the CLI and the Medical Corps. The full contingent of the CDF in the Cocos Islands numbered around 75 personnel. It was under the command of Captain George Gardiner, an accountant of an export firm in Colombo at the outbreak of war, he had obtained an emergency war commission. Two six-inch guns were deployed on Horsburgh Island, Cocos Atoll, as well as a platoon of the King's African Rifles.

The sinking of Prince of Wales and Repulse and the subsequent fall of Singapore did to British and Imperial forces what Pearl Harbor had to the Americans: compromised their ability to defend their interests north of Australia and east of India. The Japanese raids into the Indian Ocean, resulting in the loss of two cruisers and the aircraft carrier Hermes, threw Allied war plans in the entire Southwest Pacific Area into chaos.

With the Japanese successes, public sentiment on Ceylon increased in favour of the Japanese; encouraged by successful Japanese-trained and -directed rebellions in Indonesia and support for Japanese forces in Thailand, Sinkiang and the Philippines, many Ceylonese hoped that the Japanese would help them gain independence. At this time a young J.R. Jayawardene, later to be President of Sri Lanka, held discussions with the Japanese with this aim in mind. However, this was immediately stopped by D S Senanayake who collaborated with the Colonial Government, being rewarded with the Premiership, being hand-picked to lead the post-colonial government after 1948.^{[citation needed]}

On the night of 8 May, 30 out of 56 personnel of the Ceylon Garrison Artillery on Horsburgh Island in the Cocos Islands mutinied, intending to hand the islands over to the Japanese. The plan was to arrest Captain Gardiner, the British Battery Commanding Officer and his deputy, Lieutenant Stephens, disarm the troops loyal to the British Empire, turn the 6-inch guns on the CLI troops on Direction Island, and to signal the Japanese on Christmas Island, which the Japanese had occupied on 3 March 1942.

However, the soldiers all proved to be poor shots with small arms: one soldier, Gunner Samaris Jayasekera, was killed; Lieutenant Stephens was wounded by them; and the rebels' sole Bren gun jammed at a crucial moment as Gratien Fernando, the leader of the mutiny, had it aimed at Gardiner. The rebels then attempted to turn the 6-inch guns on Direction Island, but were overpowered.

Messages sent by Fernando were received in Ceylon, indicating that there was co-operation between him and both the CLI troops and the Australian signalers on Direction Island, however, none of them took part in the mutiny. The CLI helped to put down the mutineers. He declared he had surrendered on condition that he would be tried in Colombo; it may be that he intended to give a speech from the dock to inspire his compatriots. However, the 15 mutineers were court-martialed on the Cocos Islands by Gardiner. Seven of the men who were found guilty were sentenced to death, with four of these sentences commuted to terms of imprisonment. Gunner Samaris Jayasekera was buried with full military honours on Horsburgh Island on the evening of 10 May and later reburied in Singapore's Kranji War Memorial.