Mayweather, 35, who is regarded as the best defensive boxer of his generation, was freed shortly after midnight, Las Vegas police spokeswoman Laura Meltzer said. Television images showed him driving away from the jail without making any statements.
Police have said Mayweather, whose professional record is 43-0, was eligible for early release under Nevada rules that allow prisoners to leave jail early for factors that include credit for time served, good behavior and work done in prison. No details were given.
Mayweather pleaded guilty in December to a charge of felony battery and entered pleas of no contest to two counts of harassment linked to a 2010 attack on his ex-girlfriend, Josie Harris, and verbal threats against two of his children with her.
Mayweather was originally sentenced to six months but half the jail term was suspended.
He was booked into jail on June 1 after having the start of his jail term delayed in January so he could fight Puerto Rico's Miguel Cotto in a May 5 super-welterweight bout in Las Vegas.
After defeating Cotto, Mayweather discussed a possible fight with Manny Pacquiao of the Philippines, his rival for the mythical title of the world's best pound-for-pound fighter.