The 34-year-old creator is the second US streetwear star to be given the reins of a major French luxury label, after his friend Virgil Abloh, who designs Louis Vuitton menswear.

Like Abloh, Chicago-born Williams first worked with the rapper Kanye West before going out on his own with his 1017 ALYX 9SM brand.

Based in Italy, where Williams now lives, it has become so hot it got a name check alongside Nike in Canadian singer Drake's new song "Toosie Slide".

"It's been my life-long dream to be in this position, and it is really surreal that it is finally here," Williams said in a post on Instagram.

"I have worked every day for 15 years towards this single goal. 

"At the same time, it's bitter-sweet because we are living in unprecedented times and I just hope in some way I can bring hope and with my community create positive change for the industry and for the world," he added.

Williams has previously said that he owes everything to Kanye West, who is married to Kim Kardashian.

- Kanye West is his mentor -

"He is the person that gave me my first break. I created a suit jacket for him to wear to the Grammys when I was 21. He then asked me how much I got paid, to which I replied, 'Nothing.' And he said, 'Okay, I'm going to give you double nothing to come work with me.' 

"The next day I was on a plane with him to Japan. He's been an amazing friend and mentor," Williams said.

The designer later founded the cult music and fashion collective Been Trill with West, Abloh and fellow US streetwear sensation Heron Preston.

Their most notorious creation was a set of laces bearing the words "Fuck Off" that cost $100.

Sidney Toledano, of LVMH, the French fashion giant of which Givenchy is a part, said it had "watched Williams grow into a great talent.

"I believe his singular vision of modernity will be a great opportunity for Givenchy to write its new chapter."

The name of Williams' own brand comes from his daughter Alyx (pronounced "a-leeks") and his own date of birth, October 17.

- 'Something that deserves to exist' -

As well as his street cred, Williams has eco credentials from his belief that clothes should be made to last a lifetime.

"There are too many clothes on this Earth. If I am going to take the responsibility of making clothing, I need to make something that deserves to exist," he told GQ magazine.

The American replaces the British designer Clare Waight Keller, who stepped down in April after three years at the fabled, but until her arrival rather faded house.

Her biggest coup was making the wedding dress for Meghan Markle when she married Britain's Prince Harry in 2018.

Waight Keller, 49, also created a menswear range for the label that was once the favourite of Audrey Hepburn as well as bringing it back into the Paris haute couture ranks.

Williams, however, will only design Givenchy's men's and women's collections, which is sure to spark speculation that the brand is withdrawing from the elite Paris shows.

His first Givenchy show will be in Paris in October.

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© Agence France-Presse