M39 is a screw-type mount system originally designed by Oskar Barnack for Leica cameras in the 1930s. These Leica Thread Mount (LTM) camera bodies attach lenses using a 39mm threaded screw mount.

This mount was the prominent rangefinder mount for Leica until 1953 when the M3 debuted. The M Mount bayonet spelled the end for Leica’s thread mount cameras, although other manufacturers, like Canon, Leotax, and Zorki, continued to use the M39 mount for their cameras.

There are some early Soviet SLRs that also use a 39mm thread mount called M39. Lenses for this system are incompatible with M39 rangefinder lenses, and vice versa, due to differences in flange distance.

M39 rangefinder lenses can be easily adapted to mirrorless camera systems.