Floyd Mayweather Jr. -- I Guarantee Victory Over Andre Berto

Floyd Mayweather Jr. is officially confirming what TMZ Sports first told you -- he's giving Andre Berto a shot at the title next month.

Floyd just posted a promotional poster for the Sept. 12th Las Vegas bout on his Instagram.

We broke the story more than 2 weeks ago ... when the deal was actually inked.

As usual, the 48-0 Floyd is feeling confident. He boasted on his IG post, "September 12, 2015 for number 49. Come be part of the history."

Of course, Berto's no slouch at 33-3. Place your wagers.