His appointment, which had been rumoured for weeks after Slimane had dropped some heavy hints of his exit, is a calculated risk for the brand, which has seen profits skyrocket under Slimane.
After Slimane's rock star chic, Vaccarello will bring a sexy, leggy look which he has developed since he first came to fame with a collection inspired by the Hungarian-Italian porn actress turned politician la Cicciolina.
Vaccarello's "modern, pure aesthetic is the perfect fit," YSL's CEO Francesca Bellettini said.
He "impeccably balances elements of provocative femininity and sharp masculinity in his silhouettes," she added.
He "has long been recognised as one of the most talented emerging creative minds of our time," YSL's parent company Kering said.
The rumour mill has been working overtime since Slimane left, with many fashion insiders convinced that Vaccarello in his early thirties would succeed him The young designer, who comes from an Italian family, had quit Versace's Versus label earlier in the day.
Making the announcement, Donatella Versace seemed to hint that Vaccarello as going on to greater things.
"In the past several years, I have worked with three great young talents on Versus Versace, Christopher Kane, JW Anderson and Anthony Vaccarello," she said.
Scottish-born Kane's own brand has since been bought by Kering, while Irishman Anderson now heads the Spanish luxury label Loewe.
Vaccarello's typically monochrome dresses, often with plunging necklines, have an almost wild edge to them, and have won him major fashion prizes, including the top prize at the Hyeres festival.
Although his designs may be sexy and confident, he has a reputation for shyness.
He also worked for Karl Lagerfeld at Fendi before setting up his own brand in 2009 and becoming a regular fixture of Paris fashion weeks.
Slimane's departure from YSL after four years as its creative director did not come as a surprise.
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