The pair met on season 8 of the show and have defied all odds and remained together since their season aired last year.
Sharing the news of their engagement on Instagram on Sunday, the pair posted a photo of them celebrating on a boat in Sydney Harbour with a glass of bubbly.
"Forever," Johnny wrote next to a ring emoji.
Fans and other MAFS cast members shared their excitement and congratulations to the happy couple.
MAFS expert Mel Schilling said she was "so delighted".
"Such a genuine, upbeat, positive couple and you both deserve all the happiness in the world," she wrote.
"Congratulations you two," season 9 favourite Ella Ding commented.
"OMG MUM AND DAD ARE GETTING MARRIED!!!! Love love love you guys," Celebrity Apprentice star Carla from Bankstown shared.
Pair find love after divorce
Johnny and Kerry have been fan favourites since their time on the show, with the pair sharing an instant connection as soon as Kerry walked down the aisle for their TV wedding.
"As soon as I saw her bright smile… the breath just came out of me. She’s stunning, she’s incredible. I seriously can not wipe this smile off my face," Johnny said on MAFS of their first meeting.
"He put me at ease as soon as I saw him," Kerry shared. "He looks really kind. I’m stoked."
Johnny has been married before but was heartbroken when his marriage fell apart, according to Channel Nine, and had been extremely picky about who he would date.
Kerry was described by Channel Nine as "fiercely independent" and she had been married to her ex for two years before she realised she was no longer in love with him.
During their final vows on the show, they both admitted they were falling in love with one another.
They then moved in together in June last year after their series had aired.
The couple's 'first real conversation'
Johnny also posted a sweet message for their one-year anniversary, sharing a photo of the moment they had their first real conversation.
"One hour before this very moment – I was standing at the alter, my hands were sweaty, my knees shaking, I felt like I was standing there for hours. I had to whisper into my microphone (to my producer) to ask if I could go to the toilet to do a cheeky spew ( SO NERVOUS)!" he revealed.
"After a brief trip to the toilet we were back on and it was all systems go. I turned around and saw you for the first time and my life has not been the same since … I will never forget that moment. Walking down the aisle toward me, was the most beautiful woman … You smiled, reached for my hand and introduced yourself and we ALL fell in love with you.
"The day was absolute blur, a nerve-wracking, eye-opening, utterly-exhausting day but I was so excited … and energised. I felt like I had won the jackpot."
He added they finally got a brief moment to get to know each other during their "awkward" wedding shoot, and he quickly realised how "generous, warm, intelligent and stupidly gorgeous" she was.
"You have surprised me every day since this day Kerry Knight – with how much you care, how much you share, how much you love."
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