An excerpt of that letter is below:
"Wow, what a ride! I've been reflecting on my time in this great city and want to thank you for being a constant during a period of change in my life. I've experienced a few Finals appearances, a couple of championships, several weddings (including my own), the birth of four kids, bonding with an entire community and a ton of ups and downs along the way.
"It was scary leaving Toronto, a place where people really loved and supported me, and I wasn't sure if that great feeling would follow me. When I arrived in Miami, I was just hoping for glory and mention amongst the immortals of basketball. What I got was so much more."
Bosh also wrote:
"I've learned how to dream again. I've learned how to appreciate the game of basketball and all the things I've experienced even more now. People will always see the trophies and banners and think that's the whole story. But it's only a piece, only a moment in time.
"I've learned that no matter what happens on the court, the game continues. Even when things changed for me and I couldn't play, people still supported me and let me know what basketball in Miami meant for them. I truly cherish those encounters. Those good-luck chants or someone simply caring enough to ask about my health—they might seem like small gestures, but they are some of my fondest memories.
"We went through life together, Miami. You showed me how to stay strong and push through in the toughest moments. And although I didn't like it at the time, it made all the difference in the long run. It made me a better man, the person I am today. Thank you."
Bosh spoke about how the community embraced his family, learning how to persevere after losing in the NBA Finals and recovering from a six-day stint in the hospital after having surgery on his left lung.
He said of his former teammates: "The guys in that locker room always made me feel special. We helped each other through life because we were more than just teammates."
Bosh, 33, hasn't played since February 2016 due to blood clots that cost him the second half of the 2014-15 and 2015-16 seasons. An 11-time All-Star, Bosh won two championships with the Heat, famously pairing up with Dwyane Wade and LeBron James in Miami.
The Heat announced the team would be retiring his number.
"The No. 1 will never be worn by another player, and we can't wait to someday hang his jersey in the rafters," Heat President Pat Riley said in a statement, per Kevin Spain of USA Today. "Chris changed his life and career when he came to Miami...he changed our lives for the better, in a way we never would've imagined."