Annulment is a legal procedure for declaring a marriage null and void. Unlike divorce, it is usually retroactive, meaning that an annulled marriage is considered to be invalid from the beginning almost as if it had never taken place (though some jurisdictions provide that the marriage is only void from the date of the annulment). For example, this is the case in section 12 of the Matrimonial Causes Act 1973 in England and Wales.

In strict legal terminology, annulment refers only to making a voidable marriage null; if the marriage is void ab initio, then it is automatically null, although a legal declaration of nullity is required to establish this. The process of obtaining such a declaration is similar to the annulment process. Despite its retroactive nature, children born before the annulment are considered legitimate in the United States and many other countries.

The marriage annulments are closely associated with the Roman Catholic Church, which teaches marriage is a life long commitment and cannot be dissolved through divorce, but can be annulled if invalidly entered into.