Lieutenant commander (LCDR) is a junior officer rank in the United States Navy, the United States Coast Guard, the United States Public Health Service Commissioned Corps, and the National Oceanic and Atmospheric Administration Commissioned Officer Corps, with the pay grade of O-4 and NATO rank code OF-3. Lieutenant commanders rank above lieutenants and below commanders. The rank is also used in the United States Maritime Service. The rank is equivalent to a major in the United States Army, United States Air Force, United States Marine Corps, and United States Space Force.

The United States Public Health Service Commissioned Corps is the uniformed service branch of the United States Public Health Service and one of the eight uniformed services of the United States. The commissioned corps' primary mission is the protection, promotion, and advancement of health and safety of the general public.