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.