Defense of marriage amendments are U.S. state constitutional amendments that have been proposed, and in some instances adopted, to prevent the legalization of gay marriage. The term is generally associated with conservative activists favoring such an amendment.

Advocates of such amendments state that they are needed in light of the legalization of gay marriage in other countries, notably Canada, and the order of the Massachusetts Supreme Judicial Court ordering it to be legalized within Massachusetts based on that state's constitutional "equal protection" clause. Some advocates say that the "full faith and credit clause", requiring each state to recognize the actions of others, such as marriage, means that this ruling has essentially legalized gay marriage throughout the country, as those who were legally recognized as married in Masschusetts would not become otherwise merely by relocating to another state. (An analogy may be seen in divorces. A former resident of Alabama who moves to Nevada and subsequently is granted a divorce there does not suddenly become married again if they later return to or visit Alabama, even if the Nevada divorce was granted under conditions or grounds that would not have been permissible in Alabama.) Some opponents of such amendments state that "full faith and credit" obviates the effect of such amendments and notes that state laws declaring federal supremacy to be void are in themselves void. See Nullification Crisis

Some amendments and some proposed amendments go farther and forbid a state from recongizing even non-marital civil unions while others explicitly allow for this. Many such amendments were adopted in the November 2004 election and more are pending. The failure of United States Congress to pass the "Federal Marriage Amendment" has apparently led more impetus to this movement.