"From the bottom of my heart thank you for making me so happy you are going to be the best dad and husband ever. Love you,” she finished.
She revealed in a later comment that Blake designed her ring with a little help from the jewellers at Nicholas Haywood Jewellery.
Calling it her "dream ring", the reality TV star showed off the sparkling rock in a series of videos shared to her Instagram Stories.
Meanwhile her fellow MAFS alums flooded her announcement post with messages of congratulations.
Bryce Ruthven wrote "Congrats", Steve Burley said the same and Mishel Karen added: "Congratulations Beautiful girl x may you share eternal love."
Other famous friends also sent their well-wishes, including singer Samantha Jade who commented: "Congratulations beautiful lady."
Jockey Blake revealed on his own account that he asked KC to be his wife on Sunday, sharing an adorable tribute to his future bride.
"14/08/22. When the girl of my dreams / mother of my son agreed to marry me," he wrote.
"@kcosborne_ you bring me so much happiness and I'll always do my best to do the same for you. You're kind ,caring and that smile makes me light up whenever I see it.
"You make me a better man. Our little boy is lucky to have you as a mother and I'll be lucky to have you as a wife."
The pair are expecting their first child together in December this year, after KC announced the exciting pregnancy in June.
WATCH: MAFS' KC Osborne reveals dining with Bryce and his former fiancee. Story continues below.
She revealed that she and Blake are expecting a baby boy and can't wait to meet their little bundle of joy.
KC originally rose to fame when she hit Aussie TVs as a bride on season seven of MAFS, where she was paired with Drew Brauer.
Their romance didn't work out, but she later found love with Blake, the pair making it Instagram official in April 2022.
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This article first appeared on our sister site, Now to Love.