'Mardonius' was a British operation by two Norwegian agents of the Special Operations Executive against German shipping in Oslofjord in German-occupied Norway (12 March/4 May 1943).

The plan was based on the use of limpet lines, which were to be attacked to the targets' hulls by single operators working from canoes, and local volunteers were also to be trained in the work by the two SOE men, Max Manus and Gregers Gram, who were delivered by air.

While training with the Norwegian 1^st Independent Company in the UK, Manus and Joachim Rønneberg developed the initial concept for sabotage using small timed-fused magnetic limpet mines attached to the target ships by means of a long metal rod. The two men received no early response to their suggestion but the, working with Gram, Manus developed a revised and more detailed plan. This was approved, and the two men started their preparations with experimental work and training.

Manus and Gram were sent to Norway by the SOE and was parachuted into the Østmarka area to the east of Oslo on 12 March 1943. They landed near Lake Øyeren, just to the south of Tonevann, along with several containers loaded with weapons and provisions. The primary mission was to attack German shipping in the Oslofjord. The two men made a number of contacts, including Sigurd Jacobsen from Milorg, the Norwegian military resistance organisation.

The attack was made during the evening of 27 April, and Manus and Gram were aided by two local resistance personnel, Einar Riis and Halvor Haddeland. Thus there were four men in two canoes. The first was paddled by Manus and Riis to the island of Bleikøya, where the equipment for the operation had earlier been hidden, and the two men waited on Bleikøya until the fall of night, but the weather conditions were not right as visibility was good after the cloud cover had disappeared. Gram and Haddeland headed with their canoe for Ortelsburg, out of Hamburg, and attached four limpet mines, and then moved to Sarpfoss, on which they placed another four limpet mines. Manus and Riis had meanwhile paddled toward Grønlia, where they placed limpet mines on Tugela. Another possible target was Winrich von Kniprode, but any idea of attaching this had to be abandoned as the ship was illuminated by the lights of a working party.

On the following day, 28 April, the mines on Ortelsburg exploded and the ship sank within minutes. A mine left on Bleikøya also detonated, as did a mine attached to an oil lighter. Then the mines on Tugela detonated, but those on Sarpfoss failed to explode. A co-ordinated attempt to destroy ships in the Akers Mekaniske Verksted shipyard did not succeed.

After the operation, Manus and Gram departed to Stockholm in neutral Sweden and thence back the UK. Manus and Gram returned to Norway in October 1943, taking part in the SOE’s 'Bundle' undertaking. Gram was killed during a fight with the Gestapo in November 1944, and Manus survived the war.