Reading, who have taken 46 points from a possible 51 over their last 17 games, return to the top-flight four seasons after losing their place in 2008.
Fans flooded on to the pitch in celebration after a victory at the Madjeski Stadium that moved Reading, beaten by Swansea City in last season's playoff final, on to 88 points from 44 matches, three ahead of nearest pursuers Southampton who won 3-1 at Peterborough United.
"I'm delighted and It's a great night for us. It was a tough game tonight," Reading manager Brian McDermott told BBC radio.
"I couldn't see too much good in losing that play-off final at Wembley last year but perhaps this is meant to be. I'm not even bothered about the Premier League tonight, I'm just concerned about the players and staff. It's a night for the fans."
Southampton are poised to join Reading in gaining automatic promotion. The south coast club have a five-point advantage over West Ham United, who could only draw 1-1 at Bristol City on Tuesday, with two matches remaining.
The top two go up automatically and the next four teams go through to the promotion play-offs.
Mikele Leigertwood's late goal gave Reading victory over Forest who could at least take some consolation in defeat. Results elsewhere meant former European champions Forest, in 19th place on 47 points, cannot be relegated.
Portsmouth (40 points), Coventry City (40) and already relegated Doncaster Rovers (33) occupy the bottom three places.
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