Leonard didn't directly mention Toronto or the Raptors, but did say that he was "looking forward to the next chapter in my career".
Leonard was traded to Toronto last month in a deal that sent him and Danny Green to the Raptors, with guard DeMar DeRozan, center Jakob Poeltl and a protected 2019 first-round pick going back to San Antonio.
Leonard went on to thank his family, Popovich, and the fans.
Leonard - who submitted a letter to the San Antonio Express-News - thanked the Spurs organization, its fans and head coach Gregg Popovich. That year, 22-year-old Leonard became the youngest NBA Finals Most Valuable Player (MVP).More news: Students Scramble For Info After Saudi Arabia Pulls Canadian Scholarships
"The best part about San Antonio, they do right by you", the Spurs forward said, according to Dom Amore of the Hartford Courant.
"There is not a more passionate or dedicated fan base than what I've witnessed in San Antonio", he wrote.
"Through ups and downs - glad there were many more ups!"
"It's cool to see all the kids excited to hang out with us, and I'm excited to hang out with them", White said.More news: Lucas Perez posts farewell message to Arsenal
While Leonard rarely speaks publicly on his intentions, the belief among most pundits is that he wants to join the Los Angeles Lakers.
Leonard, 27, had an impressive seven-year run with San Antonio with two All-NBA First Team selections, two Defensive Player of the Year awards and a championship in 2014.
He also thanked the city of San Antonio, his former teammates and the fans.
However, questions remain about the status of a right leg injury that shelved him for all but nine games last season.More news: Kim Kardashian Wants to Change Chicago West's Name
It's likely to be a season of wondering and hoping for the Raptors, as they try to convince their quiet superstar to re-sign beyond the 2018-19 season.