Brady, arguably the most accomplished player in NFL history, believed switching teams represented a fresh challenge as his career winds down.
"For me, playing football isn't going to last another 10 years," Brady wrote. "In the time left, the question is, 'How can I keep maximizing what I do, put everything I can into it, make it the best I possibly can?' At this point in my career, the only person I have to prove anything to is myself. Physically, I'm as capable of doing my job as I've ever been.
"Now I want to see what more I can do. I want to see how great I can be. I want to hear other people say, 'Go, man. Now that's what we've been missing. That's what we need! That's what we've been looking for!' Deep down I know what I can do. I know what I can bring. Now I want to see it in action."
The soon-to-be 43-year-old also expressed gratitude toward his new team and city, and said he wanted to sign with an organization that would embrace his personality and skill set.
"Still, I'm excited," he added. "Most of all, I'm motivated. I want to deliver for my new team, my new coaches, and my new teammates. I don't want to let anyone down. I'm going to give it everything I've got. The welcome and warmth I've gotten from the players and coaches in Tampa Bay has been so gratifying. For my part, I've loved getting to know a new group of young players.
"They've welcomed me as one of their own. They want to listen to what I have to say. I'm excited to be embraced fully for what I can bring to the Bucs. In turn I'm ready to embrace fully a team that is confident in what I do - and what I bring - and is willing to go on this ride with me."
Brady also thanked the New England Patriots one final time after spending 20 years with the franchise.
"If there's one thing anyone can say for sure, it's that New Englanders understand what fandom is all about," he wrote. "New Englanders just really, really love their sports. That's maybe because compared to New York, or Chicago, or Los Angeles, Boston feels less like a big city than it does a large small town. Even if you don't know everybody in Boston, you feel like you know everybody. The fans feel like they're part of our team, and my teammates and I felt the same way about them.
"The support and love of New England fans has always been unconditional. So many great moments stand out for me - the packed training camps, the victory parades, the tens of thousands of supporters who came to see us off at the airport each time we boarded the plane for the Super Bowl. Win or lose, the same number of people would be there to greet our plane when we came home. Gillette Stadium holds around 70,000 people, and I've never not played in a sold-out stadium during my career as a Patriot. How fortunate am I?"