Judge David Fais announced the agreement on the day jury selection was to begin in a case in which Clarett was accused of holding up two people outside a bar.
Sentenced to 7 1/2 years with release possible after 3 1/2 years, Clarett also agreed to serve five years of probation after the prison sentence.
The 22-year-old said he understood he was pleading guilty and reversing his earlier plea.
"I'd like to apologize for my behavior, and I accept the time that was given to me," he said.
After the deal was announced, Clarett looked at his mother, who was sobbing and holding his 8-week-old daughter while sitting next to his girlfriend.
Franklin County Prosecutor Ron O'Brien said he was pleased with the agreement.
"Our view all along was that the conduct demanded a prison sentence, and that's been accomplished," O'Brien said. "It's in a range that will allow him to get his life back together after his release."
The concealed-weapon charge was from Clarett's Aug. 9 arrest after a highway chase with police who found four loaded guns in Clarett's sport utility vehicle. Clarett's attorneys said those guns came from his mother's house and were intended for someone else, but the attorneys did not elaborate.
Clarett, who led Ohio State to the national championship in 2002, had been charged in that police altercation with two counts of failure to comply with an order from a police officer and improper handling of a firearm, in addition to the concealed-weapon charge.
Defense attorneys said they hope Clarett can be placed in a state prison with workout facilities so he can stay in shape for a possible return to football.
"It's been a sobering experience for Maurice to stay the least. He's come to terms with certain things," attorney Nick Mango said.
Assistant Prosecutor Tim Mitchell said the plea deal is consistent with how his office handles similar cases and that Clarett did not get special treatment.
Had Clarett been convicted on all charges in both cases at trial, he would have faced three to 34 years in prison, O'Brien said.
Mitchell said he expects Clarett will serve just over 4 years, with his last six months spent outside prison at a community-based corrections facility.
The plea deal was finalized Monday morning after prosecutors met with the robbery victims. Authorities said Clarett flashed a gun and robbed two people of a cell phone early Jan. 1. One victim said all he wanted was an apology, but prosecutors were adamant about prison time, Mitchell said.
A victims' assistant from the prosecutor's office read a statement from the robbery victims, who said the ordeal has been hard on all aspects of their lives.
"Mr. Clarett, we hope you will use this opportunity to help someone along the way," the statement said.
Clarett had been drinking heavily on New Year's before the robbery, attorney Michael Hoague said. But the attorney did not explain why Clarett had a gun in his waistband.
"Obviously, that was a bad decision," Hoague said.
The case generated such intense media coverage Clarett's attorneys asked the court to pay for a public opinion poll to determine if an impartial jury could be seated for the robbery trial. Fais denied the request.
Clarett scored the winning touchdown in the second overtime in Ohio State's championship game, but that was the last time he played for the Buckeyes as a freshman.
After dropping out of Ohio State, he lost a U.S. Supreme Court case challenging the NFL's requirement that players wait three years after high school before turning pro. The Denver Broncos made Clarett a surprise third-round pick in the NFL's 2005 draft but cut him during the preseason.