Lieutenant Commander (also hyphenated lieutenant-commander and abbreviated Lt Cdr, LtCdr., LCDR, or LCdr) is a commissioned officer rank in many navies. The rank is superior to a lieutenant and subordinate to a commander. The corresponding rank in most armies and air forces is major, and in the Royal Air Force and other Commonwealth air forces is squadron leader. It is roughly equivalent to the Corvette Captain rank in central European countries and the Captain 3rd rank rank in eastern European/CIS countries.

The NATO rank code is mostly OF-3.

A Lieutenant Commander is a department officer or the executive officer (second-in-command) on many warships and smaller shore installations, or the commanding officer of a smaller ship/installation. They are also department officers in naval aviation squadrons.

Some navies address Lieutenant Commanders by their full rank or the positions they occupy ("captain" if in command of a vessel).

In the British Royal Navy, United States Navy and Commonwealth Navies however, officers are addressed by their full rank if their full name is being used, or the higher grade of the rank if the officer is being addressed or referred to in shorthand. Therefore Lieutenant Commander John Smith Royal Navy is addressed as "Commander" or "Commander Smith", and referred to as "Commander Smith".

Famous examples of this are:

- Lieutenant Commander Lionel Crabb Royal Navy who in conversation, biopics, Admiralty correspondence and even statements in Parliament by the Prime Minister is almost invariably referred to as “Commander Crabb”.

- Acting Temporary Lieutenant Commander Ewen Montagu RNVR, who is routinely referred to as "Commander Montagu" in historical and contemporary records, and in interviews by his colleagues on Operation Mincemeat.