Hai (shark) was an advanced model of the Marder-class midget submarines created in Nazi Germany during World War II and operated by the K-Verband.

Its prototype performed poorly during test runs and therefore no other boats were produced.

The Hai was 11 meters (36 ft 1 in) long and its length allowed for larger batteries, giving it the maximum speed of 20 knots (37 km/h; 23 mph) under water.

In addition, the Hai could remain under water for up to two hours.

This article about a specific naval submarine of Germany is a stub.