The 16-year-old left the show in a shock twist back in week seven - when the judges elected to keep eventual winner James Arthur in the contest instead of her.
But now she has beaten runner-up Jahmene Douglas and boy band Union J to put pen to paper - confirming the news when she appeared on Irish TV's The Saturday Night Show at the weekend.
Re-tweeting dozens of messages of support from her fans, she wrote: "Thank u for your kind messages! X It's all of you who help to inspire my music!! I cannot wait for 2013"
As the news spread the hashtag #EllaSignedToSONY became the top global trending term.
The X Factor winner James Arthur yesterday saw his debut single Impossible go to the top of the Official UK Singles Chart.
Watch MTV News on the hour every hour on MTV - Sky Channel 126 and Virgin Channel 134