The singer won selection show ‘Eurovision: You Decide’ on Friday (27 January) night, after going head-to-head with five other hopefuls.
She will now sing her song ‘Never Give Up On You’ at the contest in May.
The ballad was written by Emmelie de Forest, the Danish singer-songwriter who won the song contest in 2013.
After host Mel Giedroyc announced she had won during the live show from London’s Hammersmith Apollo, Lucie said: “I can’t believe you picked me - thank you so much”
She later tweeted: “Completely overwhelmed and so surprised. Tonight’s been one of the best nights of my life.”
Lucie, who reached the live finals of ‘The X Factor’ in 2009, fended off competition to represent the UK from fellow finalist Danyl Johnson, who appeared on the same series of the ITV talent show.
She has since made a name for herself in the West End, currently appearing in a production of ‘Rent’.
All of this year’s Eurovision contenders - Salena Mastroianni, Nate Simpson, Holly Brewer, and Olivia Garcia - had all also tried out for ‘The X Factor’ in the past.
The Eurovision Song Contest takes place in Kyiv, Ukraine on 14 May, after Jamala’s song ‘1944’ came out on top in 2016.