Paul Pierce -- Burned In Boston ... Sells Mansion for a Loss

The Paul Pierce era in Boston is now officially over -- because the former Celtics star has finally sold off his massive 2-story mansion ... but he took a pretty nasty hit on the way out.

Pierce bought the 7,631 square foot pad back in 2003 for $2.4 million ... but after he was shipped out of Boston to the Brooklyn Nets in 2013 -- Pierce put the home on the market for $2.6 mil hoping to turn a profit.

Things didn't go as planned.

TMZ Sports has learned ... after more than a year on the market, Paul finally found a buyer. But he only got $2.25 mil for the 5 bedroom, 6 bathroom spot ... $150k less than he paid for it.