Soulja Boy DROPS $55 MILLION ON PRIVATE JET

Soulja Boy doesn't know the meaning of the word recession -- because TMZ has learned, the 21-year-old rapper is dropping $55 MILLION on a new private jet.

A member of Soulja's management team tells us ... the kid bought himself a G5 jet as a birthday present to himself this week.

The jet cost $35 mil ... but that's just the beginning. We're told the birthday boy is dropping an additional $20 million to pimp out his ride with all sorts of cool stuff ... like 12 custom Italian leather seats, flat screen TVs, 4 liquor bars, a special travertine tiled floor, and Brazilian hardwood cabinets.

We're told S.B. is also gutting the in-flight lavatory -- and replacing it with a giant, LUXURIOUS bathroom. Oh, he's also giving the jet a custom paint job ... to include his logo.

And if that's not enough for someone's 21st -- we're told Soulja's also spending $300,000 to throw himself a blowout birthday party at a Miami nightclub tonight ... and Dwight Howard, Bow Wow, and Sean Kingston are all expected to attend.