Hildy Beyeler was for many years the wife and partner of the leading modern art dealer Ernst Beyeler. Together, to the surprise of Paris, New York and London, they made their “provincial” Swiss city one of the most important centres of the international art world.

No art lover is likely to forget a first visit to the Galerie Beyeler in Basle. The façade of the quiet Bäumleingasse house is bland, but the front door has a formidable air, and it is a surprise when it opens to the push. Within is what is essentially still an old thick-walled bourgeois townhouse, with low ceilings, uneven tiled floors and wooden banisters and shutters, but the white-walled rooms have been sensitively reconfigured to display the masterpieces of classic Modernist art that hang there.

It is a place where the nostalgic pull of a near-forgotten sound may draw one at last to a corner in which an elderly gentleman in a Fair Isle sweater is seated beneath a Toulouse-Lautrec preparing invoices on a manual typewriter.

At the back of the house, in what was once a little garden, there is now a double-height gallery in which will hang a few perfectly matched paintings. Just to sit there for a while, in a state of near meditation, is to learn more of modern art than could be taught in a year at any academy.

Ernst Beyeler is widely held to be the greatest dealer in modern art since the Second World War. He had intended to leave Switzerland for a job overseas, but, the war intervening, went to work with Oskar Schloss, a German Buddhist who had set up as an antiquarian bookseller in the Bäumleingasse house. Hildy Kunst was working around the corner for a silk and material merchant, and she also used to colour book illustrations for Schloss. When Schloss died in 1945, Beyeler took over the business, and fairly quickly it evolved from books to prints and drawings, and to paintings. In July 1948 Hildy Kunst and Ernst Beyeler were married, and from then on they were at one in their enthusiasm for art and for the business. As the architect Gottfried Boehm put it: “In the Beyelers’ eyes, the sensual power of art does not occupy solely a social niche — it is the very salt without which everything would lose its savour.”

Hilda, always known as Hildy, Kunst was born in Basle in 1922. She was the daughter of Ernst Kunst, a customs officer, and his wife Mina, and she was brought up in Riehen, the suburb in which she was to live for much of her married life (in a house designed for them by the noted architect Paul Artaria), and where she and her husband eventually set up their museum of modern art, the Fondation Beyeler.

According to André Kamber, the head of the Kunstmuseum Solothurn, who had begun his career as a trainee at the gallery: “She was there in the morning to empty the ashtrays, saying that running a gallery began with emptied ashtrays, and to tend the flowers. She felt responsible for everything and always protected his interests.”

The floral displays were works of art in their own right, but she was not just a gallery girl and administrator; she took an active role in decisions about paintings and policy, and shared her husband’s preference for bold and challenging art.

Pictures often went home with them, to see whether they had “durability”, and sometimes she found it difficult to let them go. Her protests ensured that many great things remained in the Beyeler Collection, which was to become the Fondation. She even threatened divorce over her favourite Picasso, the 1907 Femme, painted at about the same time as Les Demoiselles d’Avignon.

Although initially cautious, Beyeler became the doyen of the Art Basel fairs, founded in 1969, which are now among the most important contemporary art fairs in the world, with the spin-off Art Basel Miami Beach events. Sam Keller, the director of Art Basel and then of the Fondation, remembers his first impression of Hildy Beyeler in her office, receiving visitors to the gallery: she looked “like St Peter with the keys of Heaven in her hands . . . When she says something, it’s direct and clear, and you know exactly where you stand. People value her, among other things, as a good judge of human nature . . . without Hildy Beyeler the gallery would be inconceivable the way it is, and the Fondation too.” Keller was relieved to be assured that she approved the decision to appoint him.

In 1982 the couple ring-fenced their personal collection as the Beyeler Stiftung, and they set about planning a museum to house it. They had already organised sculpture shows in Riehen, and they wished to benefit their locality still more. The building, by Renzo Piano, is as beautiful as the contents are powerful, and since its inauguration in 1997 the Fondation has established itself as one of Switzerland’s most important cultural assets. It contains 200 great works of classic modernism, together with 25 carefully chosen complementary examples of tribal art from around the world — another Beyeler enthusiasm.

The Beyelers shared passions not only for art and for skiing in the Swiss mountains, but also for nature. They were early campaigners for conservation. At the beginning of their married life they had fought to save a threatened lime tree near the gallery, and later they were able to repeat the success on a much larger scale. In 1998 178 trees in the park around the museum were wrapped in a project by Christo and Jeanne-Claude. It turned out that this temporary exhibition would produce a long-term benefit.

On his 80th birthday in 2001 Ernst Beyeler wrote: “In view of the dramatic changes in the climate, art should now protect nature. For this reason, we are setting up a fund involving a broad public to afford the maximum possible protection for unspoilt forests and their populations and fauna. That means a continuation of the Amazonas campaign, with which we were able to achieve a large purchase of land for a conservation zone on the occasion of the memorable Christo tree-wrapping and the Magic of Trees exhibition . . . We are opening the fund with SFr 100,000, and will make a SFr 1 contribution for every admission to the Fondation.” In 2006 there were approximately 340,000 visitors.

Hildy Beyeler is survived by her husband of 60 years. There were no children.

Hildy Beyeler, art dealer and connoisseur, was born on July 14, 1922. She died on July 18, 2008, aged 86