ESPNcricinfo understands that McCullum, whose appearance for Toronto Nationals against Brampton Wolves in the Global T20 Canada league was his last representative match, has been signed up by the Knight Riders franchise - which owns both teams - for at least the upcoming seasons of the Caribbean Premier League and the Indian Premier League.
McCullum, 37, was expected to play in the Euro T20 Slam from August 30 as an icon player for Glasgow Giants but instead announced his retirement as a freelance cricketer. It's understood that the decision to skip the new tournament was because Trinbago Knight Riders wanted him to helm the team at the CPL this year itself, and the dates of the two competitions clashed; the CPL starts from September 5.
Indeed, in his message on Instagram at the time of announcing his retirement, McCullum had mentioned that "the next chapter, in both media and coaching, will challenge me further".
Kolkata Knight Riders recently announced that they were parting ways with Jacques Kallis, their head coach, and Simon Katich, the assistant coach who was also the head coach at Trinbago. The head coach for the IPL franchise hasn't been announced yet.
McCullum had retired from international cricket in 2016, after the Christchurch Test against Australia, but continued to turn out in franchise-cricket leagues around the world, with stints in the IPL with Chennai Super Kings and Royal Challengers Bangalore, as well as Brisbane Heat in the Big Bash, Lahore Qalandars in the PSL and Trinbago Knight Riders in the CPL.
His association with the Knight Riders goes way back. An early pathbreaker of sorts in T20 cricket, McCullum's best was the unbeaten 158 he made for the Kolkata team against Royal Challengers in the inaugural IPL game in 2008.
In December 2018, McCullum went unsold at the IPL auction. In February, he called time on his BBL career with Heat after a relatively strong season in which he made 302 runs in 13 innings. At the time, he expressed his desire to continue playing T20 cricket worldwide.
Only two weeks ago in London, McCullum was signed by Glasgow for the inaugural edition of the Euro T20 Slam, where he was to team up with Dale Steyn, among others. However, the desire to "plough on regardless" didn't appeal to him.
"I owe it to myself and the teams I represent to close that chapter rather than just plough on regardless of what I know to be true," he wrote. "In T20 cricket, I've enjoyed so many varied challenges, I can leave the game knowing I left no stone unturned."