The 55-year-old signed Leona, 30, to his label Syco after she won the third series of The X Factor in 2006.
But, despite selling over 20 million records with the company, Leona decided to part ways with them in 2014 and join Island Records.
He might be known as TV's Mr Nasty, but Simon isn't holding any grudges against talented Leona.
Speaking to The Sun, he said: "I have a real soft spot for Leona for so many reasons. I'd say to her, and I genuinely mean this, that the door will always be left open.
"Any time she ever thought, 'I want to come back to Syco' I'd welcome her with open arms.
"When you've got a talent as good as this and a voice as good as that, with the right song you can always have a hit."
Leona is gearing up to release her fourth studio album I Am, following its debut single Fire Under My Feet.
During promotional interviews for the EP, the Hackney-born singer has been open about most of the songs being focused on her bad time with Simon's label.
However Simon doesn't believe Leona was behind the comments. He continued: "She was being wound up. That just wasn't her.
"And I think she now understands that once you say something, it kind of creates your own momentum, but probably not in the right way."
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