Tom Brady Mourns Ryan Mallett's Death, 'We Lost A Great Man'

Tom Brady sent a heartfelt message to Ryan Mallett and his family after the former quarterback tragically died on Tuesday ... writing in a sweet note, "We lost a great man."

The Patriots legend -- who was teammates with Mallett in New England from 2011 to 2013 -- posted the message to his Instagram page ... adding, "Thank you for everything Ryan."

"Praying for the Mallett family and all their loved ones tonight."

The post went up only hours after cops say Mallett drowned while swimming in the Gulf of Mexico on Tuesday afternoon.

According to the Okaloosa County Sheriff's Office, Mallett was part of a group of swimmers who had been "struggling to make their way back to shore" after getting in the water at around 2:12 PM

Cops say Mallett went under ... and when he was finally pulled from the ocean, he wasn't breathing. He was later pronounced dead at a local emergency room.

The passing shocked the NFL world ... in addition to Brady, several other stars penned tributes to the ex-QB on their social media pages, including J.J. Watt -- Mallett's former teammate in Houston -- who wrote, "Gone way too soon. Rest in Peace brother."

Dez Bryant and Rob Gronkowski added messages of their own to Mallett on their Instagrams as well.

Mallett was just 35 years old.