An untitled painting by Robert Ryman on display at Sotheby’s in 2014. Mr. Ryman was one of America’s most celebrated postwar artists, known for experimenting with square canvases and white paint. (Carl Court/Getty Images)

Robert Ryman, a revered abstract artist who experimented with the most basic elements of painting — including the shape and material of a canvas, the fixtures that hold it to a wall and the movement of a brush over its surface — while working primarily with a single color, white, died Feb. 8 at his home in Manhattan. He was 88.

His death was confirmed by Susan Dunne, president of Pace Gallery in New York, which had long represented Mr. Ryman. She did not give the cause.

Wielding his brush like a prism, Mr. Ryman seemed to coax an entire rainbow out of white. Deceptively simple when seen from across a room, his canvases were generally muted squares of paint, offering little in the way of color or recognizable shapes.

On close inspection, they revealed themselves as dynamic, subtle and surprisingly moving creations, in which attention was focused on the paint itself, and on the inventive ways in which it was applied to surfaces of wood, fiberglass, steel or traditional canvas.

“It helps to think of Mr. Ryman as a kind of philosopher-carpenter with an inborn, almost mystical love of paint as paint,” New York Times art critic Roberta Smith wrote in 2015. “ ‘Is this a painting?’ ‘Is that a painting?’ could be taken as the main credo of his art,” she added. “He doesn’t always provide easy answers.”

Mr. Ryman was widely regarded as one of the most influential American painters of the last half-century. Unlike many of his peers, he was not formally trained; his artistic schooling, such as it was, occurred in the 1950s while he was working as a security guard at the Museum of Modern Art in New York. Raised in Tennessee, he had come to Manhattan to pursue a jazz career as a tenor saxophonist, and he dined on canned beans and hamburgers while taking lessons with pianist Lennie Tristano.

Mr. Ryman in 2005, receiving the Praemium Imperiale, an art prize awarded by the imperial family of Japan. (Yoshikazu Tsuno/AFP via Getty Images)

The MoMA had initially served as a practice space, where he could play in the auditorium without fear of angering other tenants in his uptown apartment. But he was soon drawn to the works of Henri Matisse and especially Mark Rothko, the abstract expressionist whose paintings were sometimes attached directly to the wall, hung without a frame.

Mr. Ryman purchased brushes and paint and, not quite sure where or how to begin, started painting in the evenings after work. By the early 1960s he was a full-time artist, covering his canvas with reds and blues only to paint over everything with white.

“At one point I just decided: Well, I’m putting this color down, and I’m really not that interested in the color that I’m putting down,” he said in a 1972 interview for the Smithsonian’s Archives of American Art. “I’m only doing it because somehow being a painter I should use color. But here I am painting it out, so why not get this down a little stronger and not put the color on in the first place?”

Mr. Ryman was often described as a minimalist, alongside artists such as Robert Mangold and Donald Judd. He preferred the term “realist,” saying that his paintings were not works of illusion, but pieces that relied on “real light and space,” varying in appearance depending on their lighting and location in a gallery.

As part of his experimentation, he worked with a sweeping array of brushes and materials, applying oils, tempera, acrylics or casein paint to surfaces that included cotton, aluminum and cardboard. Alternating mediums, he played with the way heavy and lightweight surfaces hung on the wall, or the way a piece of wood gave a brown tinge to his paint.

Robert Ryman, Untitled, 1961. (Copyright 2019 Robert Ryman/Artists Rights Society (ARS), New York)

He also tinkered with the heights of his paintings and employed unconventional fasteners, including plastic straps, aluminum tubing and strips of masking tape, which he considered no less a part of the work than the painting itself. By the 1980s, he was producing “three-dimensional paintings,” including works of painted aluminum that thrust out of the wall.

“Gritty or silky, feathery or caked, tightly-woven or unraveling, each work’s surface, like its particular cast of white and particular chromatic undertones or accents, is unique and immensely sensuous,” MoMA curator Robert Storr wrote in the catalogue accompanying a 1993-94 retrospective of his work, organized jointly with the Tate Gallery in London.

Mr. Ryman, said Dia Art Foundation Director Jessica Morgan, “pushed the boundaries of what we think a painting is.” A typical Ryman piece, she said in a phone interview, might feature unusual markings on its sides, or fixtures that are angled in different ways on each corner, or his signature — once viewed as a “passe gesture” — writ large on the canvas, or hidden in white-on-white paint.

While Mr. Ryman’s work has become a staple of permanent collections around the world, his work has sometimes baffled viewers, including at a 2010 solo show at the Phillips Collection in Washington. At times, recalled senior curator Vesela Sretenovic, his canvases triggered an all-too-common response from viewers: Well, even I could paint that.

“What I said to visitors who could not get into the work is that it’s not about stories, personal stories or specific narratives,” Sretenovic said in an interview. “It’s really hard to get people to take that in and make themselves comfortable.”

Mr. Ryman’s works, she added, were “about silence, and introspection. A silent world emerges through all these layers of paint.”

Robert Tracy Ryman was born in Nashville on May 30, 1930. His father was an insurance salesman, his mother a schoolteacher who played piano.

Mr. Ryman attended the George Peabody College for Teachers in Nashville, where he studied music but did not receive a degree. After serving in an Army band, he moved to New York in 1952 and played bebop in Greenwich Village clubs. He was hired as a MoMA guard in 1953 and was soon joined on the museum’s staff by several other emerging artists, including Dan Flavin, Sol LeWitt and Michael Venezia.

Mr. Ryman was featured in his first solo show in 1967, at the Paul Bianchini Gallery in New York, and five years later had a solo show at the Solomon R. Guggenheim Museum. More recently, he was featured in a 2015 retrospective at Dia’s Chelsea gallery in Manhattan.

Mr. Ryman’s first marriage, to Lucy R. Lippard, a MoMA colleague turned art critic, ended in divorce. In 1969, he married painter Merrill Wagner. In addition to his wife, survivors include a son from his first marriage, Ethan Ryman; two sons from his second, Will Ryman and Cordy Ryman; and six grandsons.All three of his children are artists.

While Mr. Ryman stopped short of calling himself an entertainer, he often said that pleasure — visceral or intellectual — was at the center of his work as a painter, no less than in his former life as a musician. In the MoMA catalogue, Storr quoted him as saying he wanted his viewers to enjoy “an experience of delight, and well-being and rightness.”

To stand before his paintings, Mr. Ryman said, should be “like listening to music.”