The Steelers scored a comfortable 22-13 win in front of 50,451 people at the Japan National Stadium in Tokyo on Sunday.
Rennie, who won back-to-back Super Rugby titles with the Waikato Chiefs in 2012 and 2013, had not been successful at club level since, despite a further four seasons in Hamilton, three at the Glasgow Warriors and two at Kobe before to this campaign.
His stints in Scotland and Japan were split by three seasons guiding the Wallabies between 2020 and 2022, and he has become the third former Australian boss to win in Tokyo, following on from Eddie Jones and Robbie Deans.
Rennie's attention now turns to his new role, where he will announce his maiden All Black squad on June 22, with Kobe back-rower Ardie Savea, one of the individual stars of the final, the hot tip to be the first New Zealand skipper named while playing for a club in Japan.
Sunday's game was a heart-breaking end to the Japanese career of former Wallabies flyhalf Bernard Foley.
Speculated to be on his way back to the Waratahs, where he won a Super Rugby title in 2014, the 36-year-old exited as the leading point-scorer in League One history, tallying 785 from his 73 matches.
Foley was a driving force behind the Spears' maiden Japanese title three years ago, when he finished as the competition's leading scorer with 199 points.
