As the 27-year-old actress stepped out to attend the Louis Vuitton show, all eyes were on Mary-Kate's monster rock.
According to StyleCaster, the ring in question is an intricate Cartier piece dating from 1953 — a fitting choice considering the blonde beauty's penchant for vintage clothing.
Courtesy of Sothebys and StyleCaster
The ring features a four-carat central diamond flanked by 16 sapphires, and the "petals" are set with single-cut diamonds.
It was reportedly bought at a Sotheby's auction in New York City for around £48,600, after being originally valued at between £24,000 and £36,000. The ring is said to have belonged to a Tennessee-based jewellery collector.
Mary-Kate and Olivier, who are thought to be worth £89million and £36million respectively, have been dating for two years and live in the Frenchman's townhouse in the Lower East Side of New York.
While the marriage will be the first trip down the aisle for Mary-Kate, Olivier — who is the half brother of former French president Nicolas Sarkozy — has been married before to fashion writer and author Charlotte Bernard.
The pair have two children together and divorced in 2011. A year later, Mary-Kate and Olivier went public with their romance in May 2012.
Fashionista Mary-Kate first sparked engagement rumours in January 2013 when she was spotted with a ring on her left hand during a trip to Paris, although shortly after the holiday a source told Us Weekly that the Olsen twin wanted to take her time with the relationship.
Click here for a gallery of Mary-Kate and Ashley's Throwback Thursday photos.
Like her sister, Ashley Olsen is also feeling loved-up. Last month sources suggested that Ashley, 27, was dating 47-year-old Moneyball director Bennett Miller.
The couple have been spending lots of time together in LA while Bennett makes the final cuts to his new film and were spotted on a dinner date at Dan Tana's in West Hollywood on 24 January.