Rooney, 27, fired back at Carl Foster who describes himself as season ticket holder for lifelong rivals City.
Rooney fumed: "U have more fat on ur face than i have on my body fatso. Go the gym. Fat boy."
But the plan backfired when Mr Foster bragged that his Twitter spat had gained him more followers.
He tweeted: "Got about 50 new follows after Wayne's little bite before!"