The 32-year-old Brisbane prop was diagnosed with pulmonary embolisms, or blood clots, in both lungs after playing against Wests Tigers in round seven.
Jensen was told by specialists he would be on blood thinners for the rest of his life.
The 2025 premiership winner doesn't know the cause of the clots but said a bad knock could trigger internal bleeding, making his situation worse.
"It's become clear it's no longer safe for me to continue playing rugby league. It's been incredibly difficult to come to terms with, but my health and my family's future come first," he said while tearing up on Thursday.
"As tough as today is, I don't want it to be a sad day. I want it to be in celebration of what rugby league has given me.
"If you told me as a young kid growing up in Bowen that I'd have played more than 150 NRL games, won a premiership and spent my career surrounded by incredible people, I would never have believed you."
Jensen had been training with the Broncos in the hope of receiving a clean bill of health, and even wishing he could come off the thinners for 12 months.
But the 32-year-old, who is married to wife Shannen and one of seven children, ultimately put his family first.
"I feel like if I went to my family (knowing) I made the decision to continue playing with the amount of risks that's involved in it, and something was to happen, then I wouldn't be able to live with myself, and I'd be letting them down too," Jensen said.
Jensen made his debut in 2017 for North Queensland, where he was part of the Cowboys' historic premiership run before featuring in 95 matches for Brisbane.
He also played a key role in the middle last year in the Broncos' premiership win as a powerhouse foil in the starting side with fellow middle forwards Payne Haas and Pat Carrigan.
His glittering time in Brisbane made the moment he informed his team of the devastating news on Wednesday only harder.
"To my teammates, I've watched so many of you grow from young blokes walking into the club straight out of school into husbands, fathers and genuine leaders," he said.
"I never chased individual recognition. What I've loved most is being part of a team, seeing people grow and knowing that maybe in some way I may have contributed."
Jensen is a qualified sports scientist, having completed a Bachelor of Sport and Exercise Science at James Cook University, but Brisbane coach Michael Maguire has other immediate plans for the prop.
"He was my rock. He's been the rock of a lot of things that have gone on in the organisation," Maguire said.
"He's already jumped straight into coaching, and he's still here at six o'clock in the morning and six o'clock at night, cutting clips for the players, talking to players.
"I do see him as a coach at some stage because of what I've just seen in a short period of time."
Jensen said he didn't see himself as a coach at first but has since found joy in it as he gets to help people.
But admitting his emotions were still "raw", the 32-year-old said he would decide on his future later.
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