Ana Paula Junqueira, 38, met Cowell after being invited onto his chartered yacht Slipstream with a group of friends in St Barts in January.
She was previously married to Swedish-born Johan Eliasch, the CEO of sports equipment and clothing company Head, who she met at a party thrown by supermodel Naomi Campbell.
In Tom Bower's book Sweet Revenge: The Intimate Life Of Simon Cowell, which is being serialised in The Sun, it is claimed that the pair became close "at the end of one high-spirited night" at the St Barts yacht club.
"Fuelled by champagne he invited a group of Brazilian girls on to Slipstream. By 5am he was intimately embraced with Ana Paula, a well-known international socialite.
"At teatime later that day, she had flown back to São Paulo on a private jet and had texted her intentions. 'She's coming to London soon', he announced. 'That should be fun', Cowell smiled."
Cowell took Junqueira on a date to a Chinese restaurant in London on Wednesday (April 18).
"They keep their romance low-key. Ana is very much her own accomplished woman, Simon respects her for that," a source added.
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Watch Simon Cowell speak to Digital Spy recently below: