Olsen, 27, and Sarkozy, 44 -- half-brother of former French president Nicolas Sarkozy -- first stepped out together in 2012. Since then, they've been a frequent presence at basketball games in New York, where they live together in a $6.25 million townhouse.
This will be the former "Full House" star's first marriage, and Sarkozy's second. He was previously wed to Charlotte Bernard, with whom he has two kids, Julien, 12, and Margo, 10.
Incidentally, the fashion designer's twin sister, Ashley, is also dating an older man. Following her split from David Schulte, she has been spending time with "Moneyball" director Bennett Miller, who is 20 years her senior.