The Type U 66 was a class of five submarines or U-boats operated by the German Imperial Navy () during World War I. The class is alternately referred to as the U-66-class or the Type UD. The class was built by Germaniawerft of Kiel to their 506d design as the U-7-class for the Austro-Hungarian Navy. The five boats were sold to the Imperial Germany Navy at the beginning of World War I when it was thought impossible for the submarines to reach the Mediterranean for delivery to Austria-Hungary.