In many navies, a sub-lieutenant is a naval commissioned or subordinate officer, ranking below a Lieutenant. In the Royal Navy the rank of sub-lieutenant corresponds with, but is immediately junior to, the rank of Lieutenant in the British Army and of Flying Officer in the Royal Air Force. An RN sub-lieutenant ranks above an Army Second Lieutenant or an RAF Pilot Officer.
In some armies, sub-lieutenant is the lowest officer rank. However in Brazil it is the highest non-commissioned rank, and in Spain it is the second highest non-commissioned rank.
Between 1800 and 1814 midshipmen and master's mates who had passed the examination for Lieutenant were known as Sub-Lieutenants.
Before the abolition of the rank of Acting Sub-Lieutenant in the Royal Navy, the rank of Acting Sub-Lieutenant corresponded with, but was junior, to, the ranks of Lieutenant (Army) and Flying Officer (RAF). This can be seen in print versions of STANAG 2116 edition 5 (1992).
In many Commonwealth navies (e.g. Canada and Australia) however, the rank of Acting Sub-Lieutenant still exists as a commissioned rank equivalent to Second Lieutenant, while the rank of Sub-Lieutenant is equivalent to that of an army Lieutenant. The Royal New Zealand Navy follows the U.S. precedent in titling its lowest commissioned officer Ensign.
In the modern Royal Navy, those joining as graduates start as Sub-Lieutenants with non-graduates joining as Midshipmen.
Sub-Lieutenants are confirmed in their rank and receive their commission parchments upon joining the trained strength (i.e. after Fleet Board and professional training), but their commissions are backdated to the date they were initially appointed to the rank of Sub-Lieutenant.
In Argentina, a sub-lieutenant wears a single silver sun on each shoulder.
In Brazil, a sub-lieutenant, the most senior non-commissioned rank, wears a golden lozenge.
In Mexico, the sub-lieutenant is the junior officer in the rank scale, wearing a single gold bar.
In Thailand, a sub-lieutenant and Acting Sub-Lieutenant wears a single star on each shoulder.
In the Royal Navy and the Royal Australian Navy, the insignia of Sub-Lieutenants and Acting Sub-Lieutenants consists of one medium gold braid stripe (with curl). The medium stripe should be distinguished from the narrow stripe used on the Royal New Zealand Navy rank of Ensign and the Canadian navy's Naval Cadets and in the middle stripe of Lieutenant-Commanders. The Royal Air Force followed this example of braiding when developing their rank system (see Flying Officer).
The insignia of Sub-Lieutenants looks like the United States Navy and United States Coast Guard grade of Ensign (though the United States insignia does not have a "curl"), even though its equivalent grade in the USN is actually Lieutenant Junior Grade. This is mirrored by the Canadian navy, which gives Acting Sub-Lieutenants one medium stripe and substantive Sub-Lieutenants one medium and one narrow stripe. However, in the Canadian Navy, the RN's SLt rank bar with the executive curl is worn in the #2 and #2A orders of dress (mess dress uniforms) for Acting Sub-Lieutenants and Sub-Lieutenants.