The Carthusians, also known as the Order of Carthusians (Latin: Ordo Cartusiensis), are a Latin enclosed religious order of the Catholic Church. The order was founded by Bruno of Cologne in 1084 and includes both monks and nuns. The order has its own rule, called the Statutes, and their life combines both eremitical and cenobitic monasticism. The motto of the Carthusians is Stat crux dum volvitur orbis, Latin for "The Cross is steady while the world turns". The Carthusians retain a unique form of liturgy known as the Carthusian Rite.

The name Carthusian is derived from the Chartreuse Mountains in the French Prealps: Bruno built his first hermitage in a valley of these mountains. These names were adapted to the English charterhouse, meaning a Carthusian monastery. Today, there are 23 charterhouses, 18 for monks and 5 for nuns. The alcoholic cordial Chartreuse has been produced by the monks of Grande Chartreuse since 1737, which gave rise to the name of the color, though the liqueur is in fact produced not only as green chartreuse, but also as yellow chartreuse.

In Italy the Carthusians are known as Certosini and their monastery as a Certosa.