A castellan was the governor or caretaker of a castle or keep. The word stems from the Latin ''Castellanus'', derived from ''castellum'' 'castle'.

Usually, a castellan combined the duties of both a majordomo (responsible for a castle's domestic staff) and a military administrator (responsible for maintaining defences 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. 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 often considered to be subordinates of voivodes (with the exception of the Castellan of Cracow as Cracow was the Commonwealth capital until 1596). Castellans were in charge of a part of the voivodeship called castellany until the 15th century and from that time on their domain was divided into provinces for Greater Castellans and powiats for Minor Castellans. Chancellors were district officials and had the right to attend sessions of the Polish parliament, the Sejm.