For the Patrick O'Brian novel, see Post Captain (novel).

In the British Royal Navy, Post-Captain is the rank more senior than Commander. An officer would become first a Junior Post-Captain and after three years a Senior Post-Captain.

Once an officer had reached the rank of Post, nothing but death or a court martial could stop him from reaching the rank of admiral--or so thought C. S. Forester's fictional naval hero Horatio Hornblower.

It is important to distinguish between Captains and Post-Captains. The former is a term of address used for officers in command of a ship or sloop, regardless of actual rank. These officers are addressed by their actual rank, when not in command. A Post-Captain, on the other hand, has both the rank and title of "Captain" and is always addressed as "Captain" even if he is uncommissioned (i.e., does not command a ship).

An officer took 'Post' when he was first commissioned to command a rated vessel (See Rating system of the Royal Navy); that is, a ship too important to be commanded by a mere Commander. Unrated vessels could also in some cases be commanded by Post-Captains, see 'post ship'

A junior Post usually will be commissioned a frigate or a comparable ship to command, while a Senior Post will be commissioned a Ship of the line.