"I can confirm she is engaged," Stephen Huvane said.
Swiss multimillionaire Busson reportedly gave Thurman a ring with an 8-plus carat center stone surrounded by 20 smaller stones.
Although Thurman lives in New York and Busson lives in London, the two have been dating for about a year.
It will be third marriage for Thurman, whose ex-husbands are Ethan Hawke and Gary Oldman. She has two children from her relationship with Hawke. Busson also has two kids from his prior marriage to supermodel Elle Macpherson.
The couple has not yet set a date for their wedding.