The English actress, 47, stayed submerged for seven minutes and 15 seconds, while filming a scene for the highly anticipated blockbuster sequel.
She beat Cruise, who held his breath for six minutes during Mission: Impossible — Rogue Nation.
Winslet, who plays Metkayina matriarch Ronal in the coming film, said she had video evidence of the achievement, which had been taken by her husband, who had sneaked on to the set during production.
Speaking at a global press conference ahead of the film's launch she said: "I actually have a video of when I surface from that breath-hold.
"And the only reason I have it is because my husband snuck in.
"I said, 'Please don't come because I just don't want you videoing. I'll just feel pressure, just please don't do that' and he snuck in."
She continued: "I have the video of me surfacing saying, 'Am I dead? Have I died?' And then going 'what was it?' — straightaway I wanted to know my time.
"I couldn't believe that it was 7.15 but having been told it's 7.15 you want to know what the next thing I say is? We need to radio set.
"I wanted (Cameron) to know right way, that's the first thing I wanted to do — it definitely wasn't a competition."
The Avatar sequel has reunited Winslet and director James Cameron on set for the first time since the 1997 blockbuster Titanic.
Cameron admitted: "I'm still [annoyed] because I've been a free diver for 50 years and you smoked past me and everybody else."
The original sci-fi epic from 2009 became the highest-grossing film of all time and won three Oscars for its stunning visual effects, cinematography and art direction.
It was set in 2154 and followed paraplegic marine Jake Sully, sent to the moon Pandora on a unique mission that becomes challenged when he falls for Zoe Saldana's other-worldly blue princess Neytiri.
The sequel is set more than a decade after the events of the first film and Jake has now settled down with Neytiri and their three children in the Na'vis' rainforest home.
Once a familiar threat returns, Jake, played by Sam Worthington, must work with Neytiri and the army of the Na'vi race to protect their planet.
On Tuesday, Winslet and her co-stars including Sigourney Weaver and Cameron attended the world premiere in central London for the highly anticipated sequel.
Avatar: The Way Of Water is due for release in UAE cinemas on December 15