Demi Lovato has signed on to sing the national anthem at the Mayweather vs. McGregor fight on August 26 ... Showtime has confirmed.

Lovato is a HUUUUGE fight fan. She's not only a regular at UFC fights, but also trains MMA at Jay Glazer's gym in Los Angeles.

She also just released her new single, "Sorry Not Sorry," last month -- so the timing is great for her.

Also, Diddy and Mark Wahlberg are making a $200k bet on the fight (doubling down on their Mayweather vs. Pacquiao fight bet).

Diddy has Floyd. Mark has Conor. Mark lost the first bet and paid the money to charity. Guessing it's the same plan for this fight.