In the fictional Star Trek universe, the Chief Medical Officer is a medical professional, typically a physician who is the highest ranking member of a Starfleet installation or vessel's medical staff.

This officer occupies a unique position, having complete authority over any patient in placed in his or her care even when it is a superior officer until deemed fit for duty. In addition, if in his or her professional judgement the CMO deems an person in the officer's assigned area or vessel as medically unfit, that officer can remove even a superior officer from duty until that person's condition improves.

Notable Chief Medical Officers include Leonard McCoy, Beverly Crusher, and Julian Bashir. On more recent vessels and installations, the Chief Medical Officer is supported by (or, in the case of the Starship Voyager, replaced by) an Emergency Medical Hologram.