Osaka striker Kazuya Yamamura scored the winner in the fourth minute of injury time, but with Hiroshima falling to a 1-0 home defeat to Vegalta Sendai, Toru Oniki's team secured the title with two games to spare.
After Saturday's fixtures, Kawasaki hold an unassailable seven-point lead at the top of the 18-team standings following 32 rounds, ensuring they became the first club since Hiroshima in 2013 to win consecutive titles.
A successful title defence had looked unlikely prior to the mid-season break to coincide with the World Cup in Russia as Hiroshima, winners of three J.League titles since 2012, held a nine-point lead after a strong start to the campaign.
However, Kawasaki continued to pick up points on their rivals when the season resumed and capitalised on a wretched run of form for Hiroshima, who have lost six and drawn one of their last seven games since a last win on Sept. 1.
Despite knowing a victory would guarantee the title regardless of the result in Hiroshima, Oniki's team failed to play with the attacking verve that had carried them to the top of the table.
Kawasaki's negativity was punished when their former striker Kenyu Sugimoto opened the scoring for Osaka on 55 minutes before the game burst into life in the 90th minute when Akihiro Ienaga slotted home a penalty awarded after a foul on Kei Chinen.
As the clock wound down, Yamamura pounced to score with virtually the last kick of the game, but with Hiroshima 1-0 behind to Sendai thanks to Naoki Ishihara's 67th minute effort, Kawasaki were still ready to celebrate a title victory.
Following the final whistle, the Kawasaki players stayed on the pitch to await news from Hiroshima, with Oniki and his squad finally able to savour their triumph with a vocal group of travelling fans once Sendai's victory had been confirmed.
Elsewhere, V-Varen Nagasaki were relegated after losing 1-0 to Yokohama F Marinos with Sho Ito's goal enough to leave the home side stranded at the foot of the table, eight points behind Nagoya Grampus in the 16th-place relegation playoff position.
It marked a disappointing end to Nagasaki's first campaign in the top flight after their promotion last season and they could soon be joined by 17th-placed Kashiwa Reysol, who fired coach Nozomu Kato following Tuesday's loss to Kashima Antlers.
(Reporting by Jack Tarrant; Editing by John O'Brien)
