'Jaywick' was a British and Australian special forces attack on Japanese shipping in Singapore harbour (26 September 1943).

The undertaking resulted in the sinking of seven ships by 15 men of 'Z' Force. Special Operations Australia was established in March 1942 as a joint Allied military intelligence organisation, and operated under the cover name Inter-Allied Services Department. Its personnel included several British officers of the Special Operations Executive who had escaped from Japanese-occupied Singapore, and it was these men who were the nucleus of the IASD, based in Melbourne. In June 1942, a special forces arm was organised as the 'Z' Special Unit, which was later and more commonly known as 'Z' Force, with personnel drawn primarily from the Australian army and Royal Australian Navy.

In 1943, a 28-year-old British officer, Captain (later Major) Ivan Lyon, and a 61-year-old Australian civilian, W. R. 'Bill' Reynolds, devised a plan to attack Japanese shipping in Singapore harbour. The plan called for commandos to use a vessel disguised as an Asian fishing boat to enter the harbour, where they would transfer to collapsible canoes to attach limpet mines to Japanese ships. Reynolds had a 70-ft (21-m) Japanese coastal fishing boat, Kofuku Maru. After the fall of Singapore, she had been used to rescue evacuees from ships which had been sunk along the eastern coast of Sumatra. Some 1,100 people were transported in the vessel during this period, and when the Netherlands East Indies surrendered, the vessel was sailed to India.

Lyon now ordered the boat shipped from India to Australia. Upon its arrival, he renamed the vessel Krait after a small but deadly Asian snake. In the middle of 1943 Krait, with a 'Z' Force complement of four British and 11 Australian personnel under Lyon’s command, travelled from a training camp at Broken Bay, New South Wales, to Thursday island. On 13 August Krait departed Thursday island for the US naval base at Exmouth Gulf in northern part of Western Australia. On 2 September Krait left Exmouth Gulf for Singapore with the crew disguised as Asian fishermen. After a relatively uneventful voyage, Krait arrived off Singapore on 24 September when, during that night, three officers (including Lyon) and six men left the boat in three canoes and paddled 31 miles (50 km) to establish a forward base in a cave on a small island near the harbour.

On the night of 26 September the men paddled into the harbour and attached limpet mines to several of the 45 (or possibly 51) Japanese ships before returning to their hiding spot. In the resulting explosions, the limpet mines sank seven Japanese ships and inflicted serious damage on others, the sinkings totalling more than 39,000 gross registered tons.

The men waited until the commotion had subsided and then returned to Krait, which they reached on 2 October. The return to Australia was mostly uneventful, except for an incident in the Lombok Strait when the vessel was approached by a Japanese patrol boat, but in the event Krait was not challenged. On 19 October the vessel and her crew arrived safely back at Exmouth Gulf.

The raid took the Japanese forces in southern Malaya totally by surprise and, not believing that an attack of this nature could be mounted from Australia, assumed that those responsible were local saboteurs, most likely pro-communist Chinese guerillas. In their efforts to uncover the perpetrators, a wave of arrests, torture and executions began. Malays and Chinese, as well as prisoners or war and interned European civilians were targeted in this effort.

The Allies claimed no responsibility for the attack, most probably as they wished to preserve the secret of the undertaking to facilitate future missions of a similar nature. The Japanese therefore divert no significant military resources to defending against such attacks, instead just using their secret police to exact reprisals on civilians.

'Jaywick' was followed by 'Rimau', which was less successful and very costly.