NYT | VANESSA FRIEDMAN

MILAN — In a move that could reshape the fashion world, Miuccia Prada and Patrizio Bertelli, the creative director and chief executive of the Prada Group, the thinking woman’s brand and one of the gravitational poles of the Italian industry, announced on Sunday that they were appointing the Belgian designer Raf Simons as co-creative director of Prada.

Mr. Simons and Ms. Prada will have “equal responsibilities for creative input and decision-making,” the announcement said.

The disclosure came on the final day of Milan Fashion Week — and two days after what was Ms. Prada’s last solo ready-to-wear show — at a live news conference held in the brand’s headquarters.

Miuccia Prada at the finale of the Prada fall 2020 show in Milan. The announcement on Sunday means it was her final solo ready-to-wear show. – Matteo Bazzi/EPA, via Shutterstock

Miuccia Prada at the finale of the Prada fall 2020 show in Milan. The announcement on Sunday means it was her final solo ready-to-wear show. – Matteo Bazzi/EPA, via Shutterstock

Although limited-edition collaborations have become a trend in fashion, this takes the idea to an entirely different level, especially in an industry where designers are not known for sharing the spotlight and sole aesthetic authorship is traditionally fetishized.

Mr. Simons, 52, who most recently was chief creative officer of Calvin Klein but left that post in December 2018, and Mrs. Prada said they had been in discussions for more than a year about the possibility of working together.

Mrs. Prada, 71, denied that the decision was made as preparation for her eventual retirement. Rather, both designers presented it as a reaction to the evolution in the fashion world toward a focus that prioritized commercial results over creativity, and as an attempt to right the balance.

The “fashion industry is moving more and more toward an industry that might end up excluding creatives,” Mr. Simons said during the news conference. “I see more and more the possibility of having a strong business without strong creation. That is something we do not agree with, Miuccia and myself, and I know for a fact many designers do not agree with. We think we should not forget in this business about creativity.”

Mrs. Prada said: “It is true we need to reinforce the creative aspect in the business. We like each other, we respect each other and we will see if we go somewhere.”

In an indication of just how unusual the decision was, a small group of fashion editors and critics had received a mysterious email on Saturday inviting them to a news conference on Sunday and asking them to keep the event a secret.

The Prada Group, which reported 2018 revenue of 3.14 billion euros ($3.4 billion), has been more critically acclaimed than commercially successful, with a yo-yoing share price and rumors of a possible sale to the French groups LVMH Moët Hennessy Louis Vuitton or Kering, though recent results showed signs of a turnaround. In the first half of 2019, the Prada brand — responsible for 83 percent of group sales — grew 4 percent. How the decision to double-down on the creativity might affect that trend is now the question.

Jil Sander, spring 2012 – Vittorio Zunino Celotto/Getty Images

Jil Sander, spring 2012 – Vittorio Zunino Celotto/Getty Images

Ms. Prada is well known for having joined the Italian Communist Party as a young woman in the 1960s (and protested in a Saint Laurent suit). In 1978 she took the helm of the niche leather goods company started in 1913 by her grandfather and, along with Mr. Bertelli, her husband, built it into an empire. First, with a simple black backpack in nylon and a small branded triangle on the front, introduced in the mid-1980s, that acted as a riposte to the bling and bluster that defined the fashion of the decade. Then, by introducing women’s wear in 1989, and in 1993, a second line, called Miu Miu after Ms. Prada’s nickname.

She brought ugly chic to fashion, married film noir styling to frumpiness with aplomb, and always questioned the place of fashion in the political and cultural spheres. Though she and Mr. Bertelli at one point had ambitions to build an Italian luxury conglomerate to rival LVMH and Kering — acquiring Helmut Lang, Jil Sander and the footwear brands Church’s and Car Shoe as part of the plan — they ultimately sold the two fashion lines and scaled back, focusing on the core names.

However, it was when they owned Jil Sander (which they sold to a private equity group in 2006) that they first met Mr. Simons, and took a risk on hiring him for the women’s brand. (He then was known for the men’s wear line that still bears his name, and which he will continue to own and design.)

Mr. Simons remained at Jil Sander until 2012, moving on later that year to become artistic director of Christian Dior following John Galliano’s firing after a drug-and-alcohol-fueled anti-Semitic rant in 2011.

Calvin Klein, spring 2019 – Angela Weiss/Agence France-Presse — Getty Images

Calvin Klein, spring 2019 – Angela Weiss/Agence France-Presse — Getty Images

He is known for his fascination with youth culture and an ability to marry that with an appreciation of couture tradition, often through technically challenging design. While at Calvin Klein, he won the 2017 Designer of the Year awards for women’s and men’s wear at the Council of Fashion Designers of America event — before either collection had even entered stores, almost overnight making the brand a force in New York fashion again.

However, Mr. Simons also has a recent history of short-lived stints at big brands: He left Dior after only three years, dissatisfied with the limits that its owner LVMH placed on his ability to shape the aesthetic message of the company beyond the women’s runway collection. And he had a very public falling-out with Calvin Klein management after two years over the way the ambitious direction of his designs — an attempt to redefine American sportswear with totems from Andy Warhol to the badlands of South Dakota — had disrupted sales. (Calvin Klein has since stopped holding runway shows and did not appoint a new design chief, a situation that may have helped prompt Mr. Simons’s veiled comments on Sunday about brands placing business over art.)

Both Mr. Simons and Mrs. Prada say their partnership will be the right fit. At the news conference, Mr. Bertelli characterized the pairing as “not just a professional relationship, but a human relationship. We share a similar identity.”

Raf Simons, fall 2020 – François Guillot/Agence France-Presse — Getty Images

Raf Simons, fall 2020 – François Guillot/Agence France-Presse — Getty Images

Mr. Simons said he and Mrs. Prada had been in an extended “dialogue about fashion, culture, politics, content” over the years. They share a belief that fashion needs to be about more than mere prettiness, but should grapple with issues of self and society, as well as a love of contemporary art, reflected in their personal collections as well as the Prada Foundation. Indeed, Mr. Simons said it had been his first extended creative collaboration, with the artist Sterling Ruby, that had convinced him that such a relationship could engender not conflict but rather a blossoming of ideas.

Robert Burke, founder of an eponymous luxury consultancy and the former fashion director of Bergdorf Goodman, said: “It is a strong strategic move positioning the house for the future. It will be well received by the Prada customer, and will also bring a new customer to the brand. It is a much more natural fit than the Calvin job. It could also start a trend, and perhaps an important one, since a number of brands need to plan for the future.”

Luca Solca, a luxury analyst at Bernstein, said, “All major turnarounds have been achieved by injecting new creative ideas into brands. I guess Prada would benefit from the same approach, though I wonder how this collaboration will play out.”

Neither designer would specify exact detail, though Mr. Simons said, “It is simple as when you have a lot of things to talk about. There’s more strength when two creatives talk about it and love it then when one does. When we both believe in something, we’re going to do it; when one does not, we won’t.”

What is not in doubt is that if it does succeed, it will not only create a natural heir to the Prada crown, answering outstanding questions of succession, but will also force a reckoning of sorts in the fashion world about the importance of the designer contribution. Perhaps even sparking a rebellion of sorts among other creatives, who increasingly have been treated — and seem to view themselves — as work for hire.

Along with the acclaim that greeted a one-season-only collaboration by Dries Van Noten and Christian Lacroix, who returned to fashion for it after having LVMH sell his brand in 2005 and its new owners strip it down to parts in 2009, the Simons-Prada arrangement could signal a different stage in the debate that has raged across the industry about its own value system, and whether great and unexpected products lead to great businesses or whether it is great promotion and market research that leads to great businesses, and the products are secondary.

Mr. Simons, who declined to answer questions about whether he had been given an equity stake in the business, officially begins at Prada in April. He is looking for an apartment in Milan, and will live between Italy and Antwerp, where his own company is based. Mrs. Prada will continue to design the Miu Miu line on her own.

Their first joint collection will be for spring 2021. “I am so thrilled and looking forward to September,” Mrs. Prada said.