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.

Kids these days.

USArmy He has every right to throw his money away but when he goes broke please don't bore us with tear jerking stories.

Mike Soulja Boy probably put 5% down, with no intention on making any more payments. The plane will be repossesed within 6 months.