Barcelona led 1-0 and 2-1 but Roma refused to surrender as Alessandro Florenzi, Bryan Cristante and Diego Perotti inspired a comeback.
Roma scored three goals in eight minutes as the Serie A side twice came from behind to stun Malcom and Barcelona 4-2 at the International Champions Cup.
All eyes were on Malcom after he opted to join LaLiga champions Barca, who dramatically hijacked Roma's €40million deal to sign the highly-rated Brazilian from Bordeaux last week.
Malcom – who was preparing to fly out to the Italian capital after Roma reached an agreement with Bordeaux before Barca stepped in at the last minute – was in the thick of the action on Tuesday, scoring the easiest of goals to put the Spanish giants 2-1 up.