Soul Singer Keith Sweat -- Screwed Over TWICE By Nasty Foreclosure

R&B singer Keith Sweat is singing the foreclosure blues ... because even after the bank took his house and sold it ... he's still on the hook for $250,000 -- this according to a new lawsuit.

PMI Mortgage filed the lawsuit against Sweat in Georgia -- claiming the singer defaulted on a $450,000 mortgage on his Detroit home, which he purchased in 2004.

Problem is -- the house was drowning underwater ... so when the bank sold it, it only recouped $200,000 ... a fraction of its losses.

PMI Mortgage sued Sweat for the balance -- $253,000 -- and since Keith failed to respond, a judge ruled in PMI's favor. PMI has since sought a lien on Keith's Georgia property in a last ditch effort to get its money.

Calls to Mr. Sweat were not returned.