Allen Iverson is officially mansionless.  The former NBA star's Atlanta home has been sold off in a foreclosure auction.

TMZ broke the story ... Iverson allegedly defaulted on a $1.2 million mortgage which sent the $4.5 million home into foreclosure. He briefly fought off an auction ... but ultimately, he couldn't stop it.

Sources close to the sale say Iverson's bank purchased the mansion yesterday for $2.5 million.