In Indonesia, "first lieutenant" is known as Letnan Satu (Lettu), Indonesian National Armed Forces uses this rank across all three of its services.

The first lieutenant (often abbreviated "1st Lt") in a Royal Navy ship is a post or appointment, rather than a rank.

In the Air Force, a first lieutenant may be a flight commander or section's officer in charge with varied supervisory responsibilities, including supervision of as many as 100+ personnel, although in a flying unit, a first lieutenant is a rated officer (pilot, navigator, or air battle manager) who has just finished training for his career field and has few supervisory responsibilities.

[2] As modern ships have become more complex, requiring specialized knowledge of engineering, communications, and weapons, the "first lieutenant" is the officer in command of the deck department responsible for line handling during mooring and underway replenishment.

What is known in the U.S. Navy as the "first lieutenant division" is usually composed of junior sailors (E-3 and below) who are completing their ninety days of temporary assigned duty, or TAD, that all enlisted personnel are required to perform when initially assigned to a command.

The primary mission of the division is servicing, cleaning, organizing and inventorying items within a command.

The position title of first lieutenant was held by a junior officer who was in charge of deck operations and gunnery.

The rank of first lieutenant was the equivalent of lieutenant in the current rank structure of the U.S. Coast Guard, U.S. Navy, United States Public Health Service Commissioned Corps, and National Oceanic and Atmospheric Administration Commissioned Officer Corps.