Out with the old, in with the new ... butt. 

Here's Chelsea goalkeeper Thibaut Courtois on a yacht with a smokin' hot lady friend -- just months after breaking things off with his baby mama, Marta Dominguez

The backstory ... in April, Courtois announced he and Dominguez decided to call off their relationship, despite the fact she was pregnant with their 2nd child at the time. 

25-year-old Courtois says the two are on good terms -- and now he's hanging out with new attractive women ... like the one in the tiny bikini off the coast of Spain.