圣愚（俄语：юродство），又译颠僧、佯狂者，是俄罗斯东正教的特有人物，也是一种东正教特有的禁欲主义形式。他们通常是浑身污垢、半疯、半裸体的游民传教士，脚上甚至套上脚镣，他们有些人几乎不能言语，他们的声音却被解释成神谕。圣愚（Foolishness for Christ）在英文中意义是「为了基督」的愚人。他们在人前行为疯癫，圣愚这个词暗示他们并非因过犯或者精神失调而变得如此，尽管他们的行为有些时候是故意、惹人厌恶、甚至是挑衅性的。

Foolishness for Christ refers to behavior such as giving up all one's worldly possessions upon joining a monastic order, or to deliberate flouting of society's conventions to serve a religious purpose – particularly of Christianity. Such individuals were known as both "holy fools" and "blessed fools." The term "blessed" connotes both feeblemindedness and innocence in the eyes of God.