A midshipman is an officer of the lowest rank, in the Royal Navy, United States Navy, and many Commonwealth navies. Commonwealth countries which use the rank include Canada (Naval Cadet), Australia, Bangladesh, Namibia, New Zealand, South Africa, India, Pakistan, Singapore, Sri Lanka, and Kenya.

In the 17th century, a midshipman was a rating for an experienced seaman, and the word derives from the area aboard a ship, amidships, either where he worked on the ship, or where he was berthed. Beginning in the 18th century, a commissioned officer candidate was rated as a midshipman, and the seaman rating began to slowly die out. By the Napoleonic era (1793–1815), a midshipman was an apprentice officer who had previously served at least three years as a volunteer, officer's servant or able seaman, and was roughly equivalent to a present-day petty officer in rank and responsibilities. After serving at least three years as a midshipman or master's mate, he was eligible to take the examination for lieutenant. Promotion to lieutenant was not automatic, and many midshipmen took positions as master's mates for an increase in pay and responsibility aboard ship. Midshipmen in the United States Navy were trained and served similarly to midshipmen in the Royal Navy, although unlike their counterparts in the Royal Navy, a midshipman was a warrant officer rank until 1912.

During the 19th century, changes in the training of naval officers in both the Royal Navy and the United States Navy led to the replacement of apprenticeship aboard ships with formal schooling in a naval college. Midshipman began to mean an officer cadet at a naval college. Trainees now spent around four years in a college and two years at sea prior to promotion to commissioned officer rank. Between the mid-19th and mid-20th centuries, time at sea declined to less than a year as the entry age was increased from 12 to 18.

Ranks equivalent to midshipman exist in many other navies. Using US midshipman or pre-fleet board UK midshipman as the basis for comparison, the equivalent rank would be a naval cadet in training to become a junior commissioned officer. Using post-fleet board UK midshipman for comparison, the rank would be the most junior commissioned officer in the rank structure, and similar to a US ensign in role and responsibility. In many Romance languages, the literal translation of the local term for "midshipman" into English is "Navy Guard", including the French garde marine, Spanish guardia marina, Portuguese guarda-marinha, and Italian guardiamarina. Today, these ranks all refer to naval cadets, but historically they were selected by the monarchy, and were trained mostly on land as soldiers.