In a posting on the MMA website, the 40-year-old American said he wants to face McGregor, 28, in June.
“For Conor McGregor, I’m coming out of retirement, just to fight Conor McGregor,” Mayweather told FightHype, according to the posting.
“Today, I’m officially out of retirement for Conor McGregor. We don’t need to waste no time. We need to make this… happen quickly. Let’s get it on in June.”
Mayweather had matched Rocky Marciano’s iconic 49-0 unbeaten career record when he walked away from boxing as the welterweight champion in 2015.
Speculation of a Mayweather-McGregor fight has intensified in recent months, with the US star saying he is willing to meet McGregor in a boxing ring.
McGregor, however, is under contract to the Ultimate Fighting Championship series, which would have to approve any deal for the bout to happen.
“I don’t want to hear no more excuses about the money, about the UFC,” Mayweather said. “Sign the paper with the UFC so you can fight me in June. Simple and plain. Let’s fight in June. If you want to fight, sign the paperwork, let’s do it.”
McGregor is on a break for the impending birth of his first child, a son with his long-time girlfriend Dee Devlin, who is due to give birth in May.
Last week, Mayweather taunted McGregor over his failure to agree terms on their potential superfight.
“If Conor McGregor really wants this fight to happen, stop blowing smoke up everybody’s ass,” Mayweather told ESPN, accusing the Irishman of stalling on a deal.
“Sign the paper. Sign the paper. You said you were boss, so just sign the paper and let’s make it happen.”
Speculation of a fight between retired former boxing welterweight king Mayweather and McGregor has intensified in recent months.
In February, a report in Ireland said the two sides had agreed terms and were only awaiting the resolution of a “third party hold-up” to confirm the deal.
The Nevada State Athletic Commission also confirmed it was aware of negotiations between Mayweather and McGregor.