The 23-year-old took the Olympic title from the defending champion Roberto Cammarelle, 32, despite taking up boxing only four years ago.
His medal is the third won in boxing for Britain in London 2012, following Nicola Adams and Luke Campbell. It is the best performance for British boxing in 40 years.
Joshua’s win is Great Britain‘s 29th gold medal, taking the total medal tally to 64. However the initial euphoria was dampened after the Italian camp lodged an appeal against the decision. There was a tense 10 minutes at the ExCel centre as the governing body considered the decision, but the original result remained unchanged.
The three-time world champion, Cammarelle, dominated the action in the first two rounds, leaving Joshua flagging. At the end of the first round, he was caught in the corner facing a barrage of hits from Cammarelle.
The impact of the battering carried through to the second round, with Joshua looking like he was feeling the effects.
But he came back fighting in the final round, standing his ground and landing a flurry of shots that secured the Londoner gold for Britain. Joshua rallied to launch a strong attack, backing Cammarelle into the rope, as the crowd cheered the British boxer’s name, and the final moments of attack were enough to secure Joshua the title.
The final score was 18-18, and Joshua won on countback.
Earlier in the day, Freddie Evans took silver in the men’s boxing welterweight final, losing gold medal to Serik Sapiyev of Kazakhstan.