The 39-year-old soap star - who has been playing Priya Sharma in 'Emmerdale' since 2011 - revealed the happy news on her Instagram account on New Year's Day.
The actress posted a picture of her and her husband-to-be embracing and announced to her 53,800 followers: "Two days ago this beautiful man asked me to be his wife.
"There has never been an easier answer to a question.... Yes! Feel so incredibly blessed.
"Wishing you all a wonderful 2019 & so much love for the year ahead. (sic)"
Fiona's 'Emmerdale' co-stars were quick to congratulate her and Simon, who appeared on the show when he briefly played Home Farm estate client Hugh Bryant in August 2017.
Mark Charnock - who plays Marlon Dingle - wrote: "Wonderful news!!! Congratulations lovely Fiona. What a way to go into 2019. Xx (sic)".
Michelle Hardwick - who plays Vanessa Woodfield - added: "So much love for you two."
The 42-year-old star recently got engaged herself - having proposed to girlfriend and 'Emmerdale' producer Kate Brooks last month.
Kate also added: "This is just perfect."
While Fiona's real life relationship is happy and stable, the same can't be said for her alter-ego Priya.
During her time in the Dales, Priya has had a fling with Cain Dingle (Jeff Hordley), a failed marriage to Rakesh Kotecha (Pasha Bocarie) and an affair with Pete Barton (Anthony Quinlan).
Back in March, the actress kept her new lover low-key and said she was dating an "actor and writer", but did not reveal his name.
She said: "I'm with someone right now. He's an actor and writer based in London.
"It's fairly new, we've been together since last year and he's a very wonderful guy, but I've kept it quiet."