The 'Bleeding Love' singer had been signed to Syco/Sony since winning The X Factor back in 2006, but is now leaving that label for a new contract at Universal's Island Records.
Simon sent his best wishes to Leona earlier today, tweeting: "Congratulations Leona. We had seven great years together and now wish you all the best for the future."
He added: "You put X Factor on the map."
Leona's last album was 2012 effort Glassheart, which featured the track 'Trouble'. She released a Christmas album last December, charting with infectious song 'One More Sleep'.
Her debut album, Spirit, sold more than three million copies in the UK and featured the record-breaking 'Bleeding Love', the best selling song of 2007. It's been number one in 35 countries, achieved huge sales in the US, and was nominated for two Grammys.
Congratulations Leona. We had 7 great years together and now wish you all the best for the future. You put X Factor on the map. X— Simon Cowell (@SimonCowell) June 3, 2014
Speaking about her new deal, the popstar said: "After seven incredible years at Sony I feel honoured to be given the opportunity to sign to perhaps the most iconic label of all, Island Records.
"To me Island has always been a label where artists can really flourish and are encouraged to express themselves. Island stands for quality music and signing with such a legendary label is really like a dream come true for me. I can't wait to start making music with my new team."
Island Records' president, Darcus Beese, said he'd always admired Leona's talents, and the label is looking forward to working with her.
"I've watched and admired Miss Lewis from afar, witnessed her grow into one of this country's most formidable female artists. Witnessed a girl from Hackney making and breaking records, topping the UK and US charts, earning numerous Brit and Grammy nominations, while remaining true to herself and her principles. All the while maintaining a genuine down to earth appeal.
"Soon as the ink dries, it's on to the most exciting part of all, getting into the studio and making the record we know Leona has in her."
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Watch Leona's music video for 'Bleeding Love' below: