This article is part of a series examining Responsible Fashion and innovative efforts to address issues facing the fashion industry.

PARIS — The Belgian designer Olivier Theyskens may be fashion’s greatest hoarder. For years — since his art school days in Brussels in the mid-1990s — he has kept nearly all of the fabric swatches he has ordered. “Thousands and thousands,” he said in his courtyard atelier of a regal 17th-century Paris mansion on a hot afternoon.

He wasn’t kidding. In the entrance hall, cardboard boxes overflowed with swatches. In the back, cabinet drawers erupted with more, sorted by color and texture — black crepes in one; pink laces in another. Along the walls and in the corners, skinny bolts of cloth — leftovers, or “deadstock,” from previous collections — were piled up. Everywhere one looked, there was fabric.

“I kept them because I liked them,” he said.

He doesn’t just like them. He is using them to make one-of-a-kind patchwork gowns, jackets and tunics for private clients — a sort of scrap couture that’s likely the chicest example of upcycling and slow fashion today. Mr. Theyskens not only make the clothes, but he also makes the cloth, sewing the swatches on the bias into a fabric that recalls Picasso’s harlequins. It can take several weeks to get the fabric laid out as he wants, and another few weeks to make the garment; prices run as high as $25,000 apiece.

It’s quite an about-face for designer who was once the superstar head of some of fashion’s biggest brands.

Mr. Theyskens’s pivot came during the coronavirus pandemic, when the fashion industry was suffering from an existential crisis. In May 2020 more than 500 designers, chief executives and retailers signed an open letter stating that they wanted to make the business “more environmentally and socially sustainable” by reducing output, waste and travel. Alessandro Michele, the creative director of Gucci, declared that he was moving toward a seasonless production schedule to lower the brand’s carbon footprint.

“Our reckless actions have burned the house we live in,” he wrote in his lockdown diary, which he made public.

Mr. Theyskens had a long think, too, about how to make fashion more sustainable, and he realized the answer was his fabric stash. “I wanted to produce clothes without creating waste, and optimize what we have here,” he said. “There is value to these materials.”

After much experimentation, he invented an novel process: He sews the scraps together into patchwork cloth that he treats with heat to create a crinkle effect, like Fortuny pleats, and then uses the new cloth to fashion clothes. The model Karlie Kloss walked to the Barnard College gala in Manhattan in April wearing a Theyskens maxi-dress in an appealing assemblage of blues, turquoises, violets and laces, like a stained-glass window, and she was snapped by paparazzi along the way.

“It’s a real evolutive thing,” Mr. Theyskens said. “And it has endless possibilities.”

Mr. Theyskens wasn’t always so green-minded. For nearly two decades, he was an “It” designer, known for dressing celebrities in his ethereal neo-Gothic silhouettes. He was thrust onto the international fashion scene in 1998, a year after he dropped out of school and started his business, when the Hollywood stylist Arianne Phillips put her client Madonna in one of his satin coatdress ball gowns for the Academy Awards. Critics deemed it an Oscar red carpet winner.

In 2002, Mr. Theyskens, then 25, was named creative director of the revered French couture house Rochas, and he closed his namesake brand to focus on the assignment. Though his work there drew great praise, the company’s owner, Procter & Gamble, closed the fashion division in 2006 to focus on fragrances, and he was out.

He was picked up by Nina Ricci, the old-line French ready-to-wear label, and swiftly injected it with younger, sexier vibe. Again, he was lauded by critics. But he fell out with management, and in 2009 he was let go. In 2010, Andrew Rosen, the owner of Theory, hired him as the New York company’s artistic director. That lasted four years.

Mr. Theyskens returned to Paris and in 2016 he restarted his own business. He staged a few runway shows and was selling to department stores and on e-commerce sites. Then the pandemic hit, and fashion came to a near standstill.

He moved his company to the Marais and ruminated on how he could be more eco-responsible. His resolutions: quit the relentless seasonal collection cycle and selling wholesale to focus on made-to-order couture; stop sourcing new fabrics and work with what he already had — his cache of deadstock and swatches stored at home, at the office and in his parents’ attic in Belgium.

To use the swatches, he had to separate them from their cardboard IDs, usually removing the staples with a fish knife. “It’s quite time-consuming,” he said with a quiet chuckle.

He organized the swatches by texture, color and material, building what he calls a “fabric library.”

Once sorted, he started to play with them, like collage. “Here is a selection of terrible prints,” he said, opening a drawer full of vibrant florals, zigzags, blotches and more. “But they might look good once I mix them up with something better.”

He showed off a finished piece of cloth in watery hues. “I always thought I didn’t like turquoise,” he said, “but here it is, together with lace, and it’s really nice. You never know. You clash patterns and colors, and you see something great.”

Once he has a combination that he likes, he or one of his half-dozen assistants stitch the swatches together on the bias, to give the fabric a forgiving stretch. He stews the material in a pressure cooker in his kitchen at home and creases the hot, wet mass by hand.

“You hold it together for a while, and when you let it go, you hope it’s OK,” he said. “Each one is an experiment — you have to estimate how it’s going to shrink and react — and so far, we’ve only had good surprises, no disasters. I feel there is magical good will.”

He prepares his garment prototype — or “toile,” traditionally made of muslin — by draping similarly crushed tissue paper on a mannequin in the shape of the client, like couture houses have. Using paper rather than muslin is “less wasteful,” he noted.

For the moment, Mr. Theyskens is focused on women’s wear, though eventually he’d like to do men’s wear. He has some pieces now that he describes as “nonbinary,” like a trench made from black leather he bought from a local reseller; the skin had been rejected by another fashion house because of barely perceptible flaws. He lined the coat with burgundy silk from his deadstock. The result is handsome and eco-responsible, which for Mr. Theyskens is the very definition of modern fashion.

“We must be focused on the optimization of everything,” he said as he hung the coat back on the rack. “In my mind sustainability cannot be a trend. It must be a way of being. It makes sense.”