The 1986 United States Senate election in South Dakota was held on November 4, 1986. Incumbent Republican U.S. Senator James Abdnor ran for re-election to a second term, but was defeated by Democrat and future Senate Majority Leader Tom Daschle. This was the only one of Daschle's Senate races that he won with less than 60% of the vote.
Although Abdnor won the primary by a fairly comfortable margin, it hurt him badly. Daschle was uncontested for the Democratic nomination and therefore was able to focus on the general election early, while Abdnor had to fight a challenge from a popular incumbent governor. Many political analysts say this was one of the factors in Abdnor's general election loss.