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 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.

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

On February 14, 2005, the South Dakota Senate passed, by a vote of 20 in favor and 15 against, HJR 1001, which put it on the ballot for the 2006 general election.

On November 7, 2006, at 7:00 PM CT, polls closed thought the entire state of South Dakota.