Michael Phelps' Girlfriend Megan Rossee -- Private Jet-Setting, Vegas-Hopping Hobnobber

Michael Phelps' hot cocktail waitress GF has been around the block -- in fact, she's been around the WORLD ... on private jets ... long before she ever met Phelps ... all because of her pretty face ... and TMZ has the pics to prove it.

Sources tell TMZ, 25-year-old Megan Rossee ingratiated herself with the rich and famous through her job as a model at VIP events around the country -- and thanks to her gig, she and her hot friends have been showered with lavish gifts by their elite admirers ... including expensive trips to Vegas AND flights on private jets.

We're told all the photos were taken within the last year -- in the months before Megan and Phelps eventually came into contact.