The 35-year-old died at University Medical Center, where he had been hospitalized since being injured in the fight Saturday night at the MGM Grand hotel-casino.
A hospital spokeswoman said Johnson was pronounced dead at 4:23 p.m.
Johnson was put in a medically induced coma after undergoing brain surgery less than an hour after his fight with Chavez. Doctors were initially unsure he would make it through the night, but the next day expressed cautious optimism after tests showed improvement in brain function.
Johnson was defending the IBF title he won in June against Chavez, but took a beating in the fight before finally being stopped by a flurry of punches in the 11th round.
He walked from the ring, but on the way to the dressing room began showing signs of an injury. He was rushed to UMC, where neurosurgeon William Smith operated on him to relieve pressure from a subdural hematoma, or bleeding on the brain.
His death follows the July 2 death of Mexican boxer Martin Sanchez, who died a day after he was knocked out by Rustam Nugaev of Russia in a super lightweight fight in Las Vegas. That death was the fifth in the ring since 1994 in the city.
Before Sanchez died, two other fighters suffered brain injuries but survived after fights this year in Las Vegas.