"No disrespect to Muhammad Ali, but he did it in one division," said the 38-year-old Mayweather as per BBC Sport.
"There were some other fights he lost and he's still known as 'The Greatest' because that's what he put out there. It is what it is.
"He called himself 'The Greatest' and I call myself 'TBE' (The Best Ever). I'm pretty sure I'll get criticised for what I said, but I couldn't care less."