In a decision dated Monday, US District Judge Beverly Reid O'Connell in Los Angeles said Sirhan "failed to meet the showing required for actual innocence" that might excuse his having failed to seek his freedom sooner in federal court.
Sirhan had filed his petition for habeas corpus, which could have resulted in his freedom, in May 2000.
William Pepper, a lawyer for Sirhan, did not immediately respond on Tuesday to requests for comment.
Sirhan, now 70, was wrestled to the ground with a gun in his hand after Kennedy was shot on June 5, 1968, at the Ambassador Hotel in Los Angeles, shortly after Kennedy won the California Democratic presidential primary. Kennedy died the next day.
The defendant was sentenced to death in 1969, but his sentence was commuted to life in prison after California banned the death penalty.
Sirhan was last denied parole in 2011, and is being housed in a state prison in San Diego. His next parole "suitability" hearing will be held by March 2, 2016, a spokesman for the California Department of Corrections and Rehabilitation said.
In seeking Sirhan's freedom, defense lawyers argued that he had not been physically in position to fire the fatal shot, and that a second shooter and gun may have been responsible.
The judge, however, said Sirhan's case was not strong enough.
"Though petitioner advances a number of theories regarding the events of June 5, 1968, petitioner does not dispute that he fired eight rounds of gunfire in the kitchen pantry of the Ambassador Hotel," O'Connell wrote. "Petitioner does not show that it is more likely than not that no juror, acting reasonably, would have found him guilty beyond a reasonable doubt."
In rejecting Sirhan's bid for freedom, O'Connell accepted an August 2013 recommendation by US Magistrate Judge Andrew Wistrich.
Robert Kennedy was a US Senator from New York when he died at age 42. His older brother John F Kennedy, the former US president, was 46 when he was assassinated in November 1963.