In front of over 12,000 fans at the MGM Grand Garden Arena - most of them Mexican - the fighter from Guadalajara tipped the scales at 152 pounds, despite prior speculation he would struggle to meet that limit.
Title-holder and best pound-for-pound fighter in the world Mayweather weighed in at 150.5 pounds at the precursor to the main event, with the entire arena opened up to fans for the first time in the sport's history.
Those present were soon treated to a typical piece of Mayweather showmanship as the American offered the belt to Alvarez, who declined to take it.
"I thought it would enliven the kid," Mayweather said.
Alvarez retorted: "I'm not going to let that guy tell me what to do."
Amir Khan was one of a number of high profile boxing names in attendance at the boisterous weigh-in, the Briton joined by Devon Alexander on the giant stage ahead of their IBF world welterweight title fight being confirmed next week.