Michael Phelps DITCHING GF Megan Rossee to Bro Out on Tropical Island

Michael Phelps told Bob Costas he wanted to travel the world after retiring from swimming ... and now TMZ has learned that Stop #1 is gonna be AWESOME ... a boys trip to one of the most expensive tropical islands in the world!!

Sources close to the couple tell TMZ, Phelps is planning to visit the Maldives with several close male friends in the near future -- a notoriously expensive island destination for the rich and famous ... so obviously, it's amazing.

But here's the rub -- we're told one person was NOT invited ... Michael's hot GF Megan Rossee.

According to sources, Michael wanted to do a boys' trip after his record-setting Olympic Games in London -- so he's going without the ol' ball and chain.

(FYI -- the Maldives is an archipelago located in the Indian Ocean, and is Asia's SMALLEST country. Sitting at just 4'11" above sea level, it's also the world's LOWEST country. The more you know.)

But Phelps hasn't completely forgotten about his one and only -- we're told Michael promised to make it up to Megan by taking her on a separate trip ... just the two of 'em ... anywhere in the world.

Not a bad consolation prize.