军舰旗（FIAV符号：）是军舰悬挂以表示所属海军的海上旗帜，同时亦表明了船只的军用身份，通常在舰尾或主桅悬挂。在航行及日出至日落的停泊期间均会悬挂军舰旗。部分国家的海军在作战中往往会同时在桅杆上悬挂几面船旗，因为传统上认为降下船旗代表投降。

根据《联合国海洋法公约》第20条约定潜水船及其他水中航行机器，在沿岸国的领海时必须在海面上航行并悬挂起所属的旗帜。对潜艇而言，如悬挂起军舰旗则与其他军舰无异，在沿岸国的领海时无损其通航权。

政府船只（如海岸警卫队）及商船另有设计商船旗，若干海军另有制定舰首旗，悬挂在舰首。

An ensign is, in its widest sense, a flag or other standard. In particular, ensign has commonly been used to mean the flag or other standard of a ship or military unit.

Today, the term is most commonly used in reference to ships, where the ensign is the largest flag, generally flown at the stern (back) of the ship. Naval ensigns today do indicate the ship's (nominal) nationality, but may be a specifically naval ensign rather than the normal national flag.