Matty Blair fired the visitors ahead but the Spireites hit back after the break with two goals in four minutes from Sam Hird and Gary Roberts to win the league.
Play-off bound Fleetwood made five changes but they struck first when Blair played a one-two with Josh Morris before sweeping a low shot past goalkeeper Tommy Lee from 10 yards.
Eoin Doyle had a chance when he got in down the Fleetwood left five minutes later but he dragged his shot wide, and Chesterfield were almost caught out again in the 41st minute when Gareth Evans broke away but his shot was deflected behind for a corner.
However, the hosts turned the game around with two quick-fire goals at the start of the second half.
First Hird headed Jay O'Shea's cross in from eight yards out in the 51st minute and then Roberts curled a left-foot shot past keeper Chris Maxwell moments later.
Fleetwood rarely threatened to spoil the party after that and Chesterfield could have scored more goals before the final whistle sparked off wild celebrations among the 9,000-plus crowd.