The 2006 winner was signed by Cowell's Syco wing of Sony music after capturing the nation's heart with her impressive voice and modest personality.
With her latest album Christmas with Love, selling only 100,000 copies, the 29-year-old has now moved on to Island Records, which boasts artists including Drake, Robbie Williams and Nicki Minaj.
"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," Lewis said in a statement.
"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 added: "We are absolutely delighted to welcome Leona into the Island family.
"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.