U-25 was a Type 1A submarine built for the Kriegsmarine during the years of 1935 to 1936. The Type 1 was Germany's first post WWI attempt at an oceangoing submarine. There were on two ships of this class built.

U-25 served between the years of 1936 to 1940.

U-25 was used primarily as a training submarine, due to her being slow to dive, and her poor performance. However, with Germany's shortage of submarines, she was pressed into active service. Between October 1939 and the end of June, 1940 U-25 had sunk 8 vessel, and partaken in one Wolf Pack. She has one notable action:

On 17 January 1940, 10 miles north of Shetland, U-25 torpedoed SS Polzella. Enid, of then-neutral Norway en route to Dublin, went to assist Polzella. U-25 then shelled and sank Enid. Her crew escaped in their lifeboats. None of Polzella 's crew survived.