The court ruled 4-1 against Skakel's request.
Skakel - a nephew of Robert Kennedy's widow, Ethel - was sentenced to 20 years to life in prison in 2002 for fatally beating Martha Moxley with a golf club in 1975 in a wealthy Connecticut suburb. Monday's decision came after years of appeals and a campaign by Skakel's cousin, Robert Kennedy Jr.
Skakel, 49, had asked for a new trial after Gitano "Tony" Bryant, who attended the same private school as Skakel, implicated his two friends in the killing. A judge turned that request down in 2007, and Skakel then appealed to the high court.
Bryant gave a videotaped statement to an investigator hired by Skakel in which he said his two friends were in Greenwich the night Moxley was killed. He said they told him they got Moxley "caveman style."
Bryant has since invoked his Fifth Amendment right against self-incrimination. The two men he implicated have done the same.
Prosecutors have said Bryant's claim was fabricated and that nobody saw him and his friends in the predominantly white, gated neighborhood the night of the murder. Bryant and one of the men he implicated are black; the other has been described as mixed race.
In 2006, Skakel lost an appeal before the state Supreme Court in which he argued, among other things, that the statute of limitations had expired when he was charged in 2000. He still has an appeal pending in federal court.
