4:14 PM PT -- Tom Brady has responded to Gronk's announcement with a heartfelt message. He posted a photo of the two of them on the field, and added a caption that read ... "What an honor and privilege to play with you these past nine years @gronk! You accomplished so much and our team was almost unbeatable when you were on the field! Your fun-loving, inspiring, and positive energy made an impact on everybody you came into contact with! Not just that, but for as great as a player you are, you are a better teammate and person. The NFL was a better place with you in it! But I have no doubt you will be a success in anything you do!!! Love you pal!"

New England Patriots star Rob Gronkowski says he's finally hanging it up and retiring from football at age 29 ... this after months of speculation.

The longtime tight end broke the news on Instagram Sunday with a lengthy caption attached to a shot of him holding the Lombardi Trophy. He wrote, "It all started at 20 years old on stage at the NFL draft when my dream came true, and now here I am about to turn 30 in a few months with a decision I feel is the biggest of my life so far. I will be retiring from the game of football today."

He goes on to thank Patriots owner Robert Kraft and his head coach, Bill Belichick, for the chance they gave him when "drafting my silliness in 2010." Gronk continued to reflect on his time with the Pats' organization, saying ... "My life experiences over the last 9 years have been amazing both on and off the field. The people I have meet, the relationships I have built, the championships I have been apart of, I just want to thank the whole New England Patriots organization for every opportunity I have been giving and learning the great values of life that I can apply to mine."

Gronk then went on to thank Pats fans for accepting him for who he is, while adding that it's time to move on knowing Patriot Nation is a huge part of his makeup. He ended with one last thank-you to his teammates -- past and present -- by saying, "To all my current and past teammates, thank you for making each team every year special to be apart of. I will truly miss you guys. Cheers to all who have been part of this journey, cheers to the past for the incredible memories, and a HUGE cheers to the uncertain of whats next."

Rob is a 3-time Super Bowl champ with the Pats, a 5-time Pro Bowler and has made First-Team All-Pro 4 times. He's widely seen as a shoo-in to get into the NFL Hall of Fame one day as well.