Dexter Isaac has come forward in the case, claiming that he was paid $2,500 (£1,541) by notorious gangster James "Jimmy Henchman" Rosemond to rob Shakur at Quad Studios in New York City in November 1994.
Isaac, who is currently serving a life sentence behind bars for unrelated offences including robbery and murder, issued an apology to Shakur's family via AllHipHop.com on the eve of what would have been the late rapper's 40th birthday.
"I want to apologise to [Shakur's] family and for the mistake I did for that sucker [Rosemond]," Isaac said. "I am trying to clean it up to give [Shakur and Notorious B.I.G's] mothers some closure.
"In 1994, James Rosemond hired me to rob Tupac Shakur at the Quad Studio. He gave me $2,500, plus all the jewellery I took, except for one ring which he wanted for himself. It was the biggest of the two diamond rings that we took.
"He said that he wanted to put the stone in a new setting for his girlfriend at the time, Cynthia Ried. I still have as proof that chain we took that night in the robbery."
Because of the statute of limitations, Isaac cannot be prosecuted for the offence.
He has said that he decided to confess to the shooting after he was accused of being a government informant by Rosemond in a May 2011 statement.
Shakur made a recovery from the 1994 shooting, but was shot and killed in September 1996. Notorious B.I.G, who was one of a number of individuals apparently accused by Shakur of involvement in the 1994 incident, was killed in March 1997. Neither murder has been solved.
A spokesperson for the New York Police Department said that officers are looking into the statement and would seek to interview Isaac if its authenticity is verified.
Shakur was recently reported to be alive and living in New Zealand in an article posted but later removed from the PBS website.
Producers are currently searching for an actor to play rapper Shakur in a biopic based on his life.