Before his victory over McGregor via a 10th-round stoppage on Saturday night (26 August), "Money" had appeared on an episode of Jimmy Kimmel Live, revealing that he would be betting on himself.
According to ESPN, Mayweather followed through as just a couple of hours before the big fight, he went into the sportsbook at the M Resort in Las Vegas to place a $400,000 (£310,200) bet.
The wager was on the fight ending in under 9.5 rounds at 1/2 odds, however, it was not able to go through as there were concerns over the legality of the bet as Mayweather was betting on something other than a straight win.
With the delay that was caused by his initial bet, Mayweather then wanted to bet on a knockout, but grew impatient as he had to wait to place another bet and eventually left.
Interestingly, the former five-weight champion would have made money from either bet if they went through as he defeated the Irishman early on in the 10th round, which counts as winning under 9.5 rounds, while he did finish McGregor even if it was not a clean knockout.
However, Mayweather claims he sent his "guy" to bet on the fight ending in under 9.5 rounds, giving him six-figures to place a wager on.
"I think that we bet 100 [thousand] on 9½," Mayweather told ESPN after the fight. "I gave my guy six figures to go bet. I have to make sure he bet because earlier today [Saturday] I went to the sports book to bet and they wouldn't let me bet."
The 40-year-old later told SportsCenter that he gave a friend $400,000 to bet, but that he was only allowed to place a bet for $87,000 (£67,500).