A castellan was the governor or captain of a castle. The word stems from the Latin Castellanus, derived from castellum "castle". Also known as a constable, governor of the castle or captain.

Usually, a castellan combined the duties of both a majordomo (responsible for a castle's domestic staff) and a military administrator (responsible for maintaining defenses and protecting the castle's lands). This was particularly the case if there was no lord resident at the castle, or if the resident lord was frequently absent.

In France, castellans (known in French as Châtelains) who governed castles without resident nobles acquired considerable powers, and the position actually became a hereditary fiefdom.

In Germany the castellan was known as a Burgmann, or sometimes Hauptmann ("captain"), who reported to the lord of the castle, often known as the burgrave (Burggraf).

At times, there was a castellan among the Officers of the Kingdom of Jerusalem.

In the Kingdom of Poland and later the Polish-Lithuanian Commonwealth, the Castellans (Polish: Kasztelan) were in most cases lower in precedence to the voivodes (with the exception of the Lord Castellan of Kraków who had precedence before voivode of Kraków). Castellans were in charge of a part of a voivodeship called Castellany (Polish: Kasztelania) until the 15th century and from that time on their domain was divided into provinces for Greater Castellans and powiats for Minor Castellans. Castellans in the Polish Lithuanian Commonwealth were of senator rank.