A flag officer is a commissioned officer in a nation's armed forces senior enough to be entitled to fly a flag to mark the position from which the officer exercises command.

The term is used differently in different countries:

• In many countries, a flag officer is a senior officer of the navy, specifically those who hold any of the admiral ranks; the term may or may not include the rank of commodore.
• In some countries, such as the United States, India, and Bangladesh it may apply to all armed forces, not just the navy. This means generals can also be considered flag officers.
• In most Arab armies, liwa (Arabic: لواء), which can be translated as flag officer, is a specific rank, equivalent to a major general. However, "ensign" is debatably a more exact translation of the word. In principle, a flag officer commands several units called "flags" (or "ensigns") (i.e. brigades).