Portelet Tower is built on a small Island, known as Ile de Guerdain, in Portelet Bay, Jersey, Channel Islands.

The tower is popularly known as Janvrin's Tomb because sea captain Philippe Janvrin was buried there after he died of the plague, but this was some considerable time before the tower was built.

The tower consists of only one room and was more of a guardhouse than a Martello. It has a door at ground level and one window about seven feet above the ground level. It is built of rubble stone and measures 17 ft (5.2 m) in height and 27 ft (8.2 m) in diameter.

After its completion in March 1808, Lieutenant Governor George Don ordered that coal be brought that a fire might be burned in it day and night for two weeks to dry it out.

A garrison consisting of a sergeant and 12 men then occupied the fort, which was armed with an 18-pounder carronade.

It is styled on the Martello design and was completed by March 1808, when the Lieut-Governor, General Don requested that it be dried out for a fortnight and then occupied by a sergeant and twelve men.