The fight began cautiously as both men weighed each other up. However, it sparked into life when Cammarelle pinned Joshua in the corner in the final moments of the first round before unleashing a torrent of scoring punches.
It was still an even contest in the second round as Cammarelle continued to bide his time and waited to launch his attacks.
The Italian led by three as they entered the final round. Joshua launched everything and produced a spirited finish as he battled to keep his gold-medal dream alive.
The judges scored the bout level, but it was Joshua who prevailed on countback to claim gold for Great Britain.