The M15 class comprised fourteen monitors of the Royal Navy, all built and launched during 1915.

This resulted in the first four of the class, which were built by William Gray & Company of Hartlepool, receiving the Mk X mounting.

The remaining ten ships, all built by Sir Raylton Dixon & Co., Middlesbrough, all received the Mk VI mounting.

While generally positive about the performance of the design, he noted that they had a tendency to roll, using a specific occasion with M25 as an example; M25, M26, M27 and M28 served in the Dover Patrol from 1915 to 1918.

The remainder served in the Mediterranean from 1915, with M23 joining the Dover Patrol in June 1917 and M21 in October 1917.