2006 South Dakota Amendment C

South Dakota Amendment C of 2006 is an amendment to the South Dakota Constitution to make it unconstitutional for the state to recognize or perform same-sex marriages, or to recognize civil unions, domestic partnerships, or other quasi-marital relationships regardless of gender. The referendum was approved on 7 November 2006 by 52% of the state's voters.

The text of the adopted amendment states: "Only marriage between a man and a woman shall be valid or recognized in South Dakota. The uniting of two or more persons in a civil union, domestic partnership, or other quasi-marital relationship shall not be valid or recognized in South Dakota."

The amendment was rendered void by Obergefell v. Hodges, a US Supreme Court decision that legalized same-sex marriage nationwide.