Frankfurt battled hard, with keeper Ralf Fahrmann saving two penalties, and even took the lead through Sebastian Rode before Lucas Barrios scored twice.
Marco Russ added an own goal to give the hosts a winning finale and kick off a weekend party in front of 80,720 home fans in the stadium and thousands more across the Ruhr valley city.
"This is undoubtedly a great day for Borussia and May 14 will go down in history," a beaming Dortmund coach Jurgen Klopp told reporters. "There is such a contrast of emotions, us celebrating the title and Eintracht being relegated."
German league president Reinhard Rauball, who is also Dortmund boss, handed over the trophy to keeper and captain Roman Weidenfeller in front of another sold-out crowd as a sea of yellow confetti enveloped the Westfalen stadium.
The fans swarmed the pitch a little later as the players brought the trophy into safety behind a long line of stewards and Klopp was doused with beer by his players.
Some 400,000 people are expected to take part in Sunday's title party in the city with the youngest Bundesliga winning team set to tour the streets on a bus to show off their seventh German league trophy.
Bayer Leverkusen held on to second place with a 1-0 victory at Freiburg to book their automatic Champions League qualification, leaving Bayern Munich in third and with a spot in the Champions League qualifying rounds despite their 2-1 win over VfB Stuttgart.
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There was some consolation for Bayern in a disappointing season, however, with striker Mario Gomez clinching the top scorer trophy after grabbing the equaliser to take his league tally to 28.
"There is no reason for big celebrations because we just met our minimum aim," said Bayern's interim coach Andries Jonker with the Bavarians desperate to have a shot at the Champions League with the final to be held in Munich next year.
Hanover 96 and Mainz 05 will compete in the Europa League after finishing fourth and fifth respectively.
At the other end of the table, Felix Magath's VfL Wolfsburg, the 2009 champions, stayed up after going through a rollercoaster of emotions in coming from a goal down to beat hosts Hoffenheim 3-1 thanks to two goals from Mario Mandzukic.
Borussia Moenchengladbach completed their spectacular comeback, drawing at Hamburg SV 1-1 to capture the relegation play-off spot with three wins and a draw in their last four games.
St Pauli will join Frankfurt in the second division.
Frankfurt, who began the day a point behind Wolfsburg, looked determined to make the best of their situation and Faehrmann saved a 12th minute Barrios penalty to boost their hopes before Rode tapped in a low Fanis Gekas cross from two metres out after the restart.
With Wolfsburg trailing at that time against Hoffenheim Frankfurt had thoughts of a great escape but Paraguay international Barrios fired in the equaliser an
