Clean Monday (Greek: Καθαρά Δευτέρα), also known as Pure Monday, Ash Monday, Monday of Lent or Green Monday, is the first day of the Eastern Orthodox Christian and Eastern Catholic Great Lent. It is a movable feast that occurs at the beginning of the 7th week before Orthodox Easter Sunday.

The common term for this day, "Clean Monday", refers to the leaving behind of sinful attitudes and non-fasting foods. It is sometimes called "Ash Monday," by analogy with Ash Wednesday (the day when the Western Churches begin Lent). The term is often a misnomer, as only a small subset of Eastern Catholic Churches practice the Imposition of Ashes. The Maronite Catholic Church is a notable Eastern rite that employs the use of ashes on this day.