Rear Admiral is a naval commissioned officer rank above that of a Commodore and Captain, and below that of a Vice Admiral. It is generally regarded as the lowest of the "Admiral" ranks, which are also sometimes referred to as "Flag officers" and/or "Flag ranks". In many armed forces it is referred to as a two-star rank.

It originated from the days of Naval Sailing Squadrons and can trace its origins to the Royal Navy. Each Naval Squadron would be assigned an admiral as its head, who would command from the centre vessel and direct the activities of the squadron. The admiral would in turn be assisted by a vice admiral, who commanded the lead ships which would bear the brunt of a naval battle. In the rear of the Naval Squadron, a third admiral would command the remaining ships and, as this section of the squadron was considered to be in the least danger, the admiral in command of the rear would typically be the most junior of the squadron admirals. This has survived into the modern age, with the rank of rear admiral the most-junior of the admiralty ranks of many navies.

In some European navies (e.g., that of France), and in the Canadian Forces French rank translations, the rank of rear admiral is known as counter admiral. In the Royal Netherlands Navy, this rank is known as schout-bij-nacht, (lit.: supervisor during night), denoting the role junior to the squadron admiral, and fleet admiral.