The 31-year-old from Sunderland - who had previously been tipped to win the final - beat Rory Taylor, who came second in the public vote, and Roger Wright, who came third.
Forster had sung 'Who Wants to Live Forever?' by Queen as his final performance in front of the judges, before joining the other two remaining contestants for a rendition of 'Gethsemane (I Only Want To Say)' from Jesus Christ Superstar.
After being awarded the role, Forster sang a rendition of 'Hosanna' with the other finalists.
Forster will now join Mel C, Tim Minchin and Chris Moyles in a nationwide tour of the musical.
The tour dates for Jesus Christ Superstar are as follows:
Friday 21 September - London O2 Arena
Tuesday 25 September - Glasgow SECC
Friday 28 September - Newcastle Metro Radio Arena
Sunday 30 September - Manchester Arena
Tuesday 2 October - Cardiff Motorpoint
Thursday 4 October - Birmingham NIA
Tuesday 9 October - Belfast Odyssey (on sale 24th May at 9am)
Friday 12 October - Dublin O2 (on sale 24th May at 9am)
Tuesday 16 October - Liverpool Echo Arena
Friday 19 October - Nottingham Capital FM Arena
Sunday 21 October- Sheffield Motorpoint
