Leicester, who beat Sheffield Wednesday 2-1 on Friday after goals by Riyad Mahrez and Anthony Knockaert, have 89 points from 40 matches, and are guaranteed to finish in the top two positions in the second-tier Championship after results went their way.
QPR lost to Bournemouth 2-1 while Derby went down 1-0 to Middlesbrough, meaning neither side can overhaul Nigel Pearson's side with six matches remaining.
Leicester are unbeaten in 21 matches, and lead second-placed Burnley by nine points.
Goalkeeper Kasper Schmeichel, son of former Denmark and Manchester United great Peter, said on Twitter: "Premier league! So proud of all the boys and everyone at the club!!!"
Promotion to the Premier League brings vast financial rewards, with a 2013 report by the Sports Business Group at Deloitte saying it could be worth up to £120 million, much of that from broadcast rights.
Even immediate relegation triggers parachute payments of around £60 million over four years.
Leicester, owned by Thai businessman Vichai Srivaddhanaprabha, missed out on automatic promotion last season after winning only two of their final 13 matches before being knocked out in the playoffs by Watford after Knockaert missed a last-minute penalty and their opponents scored at the other end in a dramatic finish.
They have lost only five matches this season, going from strength to strength on the back of leading scorers David Nugent (18) and Jamie Vardy (16), and after numerous managerial changes since their last stint in the Premier League, have found a level of consistency under Pearson.
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