Unsuspecting Charlotte Malton, aged 27, climbed into the ring at a charity match at Doncaster Racecourse, thinking she had won a raffle for a bottle of prosecco.
But in front of a crowd of around 400 people, Dean, 29, joined her then knelt down to ask her to marry him.
The worker at Enns and Sons in Doncaster engineered the whole surprise with event organiser Tom Digby.
As Dean’s sister Kelly MacLane was boxing on the night, his parents and many friends were in the audience to witness the romantic moment.
Mr Platts said: “Charlotte didn't have a clue. Neither did my parents – it was a total surprise.”
The ultra white Collar Boxing event allowed participants eight weeks of free professional boxing training, prior to the showcase event.
Those who take part are asked to sell tickets and raise at least £50 for Cancer Research UK.
Carer Mrs MacLane, 31, lost her fight, but raised £300 for the charity.
Vehicle engineer Dean has taken part in UWCB bouts so had some experience of being in a boxing ring, but admitted: “It was nothing like this.”
He signed up with UWCB for the thrill of being the ring, but also to raise money in memory of his grandfather, James Stevenson, who died of cancer.
He joined Kelly at training to give moral support, then decided to propose to Charlotte at the show, knowing his parents, cleaner Mandy and taxi driver Ian Platts, would be there.
For the proposal, Mr Platts wore a red UWCB vest with the words ‘Will you marry me??’ printed on the front, and had a matching blue ‘I said yes!!’ vest made for Miss Malton.
He added: “When I went down on one knee there was a huge cheer from the audience and applause – it was very loud.
“I think Charlotte wanted to cry, but she didn't because of the crowd. I think the shock took over really.
"She was quite embarrassed, because it was in front of everybody.
“She was calling me an idiot, but she said yes.”
The pair, who live on Chatworth Crescent, Scawthorpe, plan to get married in Cyprus in 2022.
Mr Platts met Miss Malton, deputy manager at Danum Daisies Nursery, on a Christmas party night in Jack's Bar, Doncaster, in December 2018.
Mr Platts said: “I looked around the room and she just stood out, so I thought ‘you know what, I’m going to talk to her’. And now we’re engaged.”
Of his experience with UWCB, he said: “The whole event was great. I was very proud of my sister - she was giving it her all and it was for a good cause.
“Ultra has been amazing – I’ve made a lot of friends I keep in touch with on Facebook, but I’ve also got a boxing family – we all have that bond.
“It’s amazing being in the ring – I’m going to sign up for the next bout in November.”
The next Ultra White Collar Boxing event takes place in Doncaster on November 16, 2019, and training starts on September 23.
Anyone interested should visit the website: https://www.ultrawhitecollarboxing.co.uk/signup/