Captain in the U.S. Army (USA), U.S. Marine Corps (USMC), U.S. Air Force (USAF), and U.S. Space Force (USSF) (abbreviated "CPT" in the USA and "Capt" in the USMC, USAF, and USSF) is a company-grade officer rank, with the pay grade of O-3.

In the British Army, the captain was designated as the appropriate rank for the commanding officer of infantry companies, artillery batteries, and cavalry troops, which were considered as equivalent-level units.

On July 23, 1775, General Washington decreed that captains would wear a "yellow or buff" cockade in their hats as their badge of rank.

Marine captains also serve as executive officers (i.e., second-in-command) of infantry battalion weapons companies and some other larger combat logistics and aviation support units.

Other health care professions including nurse anesthetists, pharmacists, optometrists, veterinarians, and physician assistants, among others may start as first lieutenants promotable upon completion of initial entry training.

In Army and Air Force Judge Advocate General Corps, lawyers with a Juris Doctor degree and membership in the bar of at least one U.S. state or territory are appointed captains, or first lieutenants promotable upon completion of initial entry training.

However, in Marine Corps Judge Advocates, after earning their commissions as unrestricted line officer second lieutenants, as well as earning an accredited Juris Doctor degree and passing a bar examination, enter active duty as first lieutenants and must complete the Marine Officer Basic Officer Course/The Basic School to qualify as rifle platoon commanders, before subsequently attending their MOS school prior to assignment to their first Marine Corps Judge Advocate billet and completing the minimum time in grade requirements for selection for and promotion to captain.