Claude Bolling, a French pianist, composer and bandleader who became one of the most successful jazz musicians in Europe and gained a devoted following across the Atlantic with his pleasing fusions of the jazz and classical traditions, died Dec. 29 in Garches, a suburb of Paris. He was 90.

His death was announced on his website, which did not provide other details.

A devotee since childhood of Duke Ellington, Fats Waller and other eminences of American jazz, Mr. Bolling grew up listening to their music on the radio until World War II intervened. “Jazz was all but banned by the Nazis in my country,” he told the Hartford Courant. “So I got most of my jazz from 78 rpm recordings.”

Mr. Bolling said Ellington took him in “as part of his family” when they met in the 1960s, by which time the Frenchman had embarked on his career as a bandleader. Describing the effect of Mr. Bolling’s music, trumpeter Louis Armstrong was reputed to have declared that “my heart will never forget the sounds he made.”

Those sounds were not daring — Mr. Bolling generally hewed to traditional jazz — but they won him steady audiences in Europe and the United States for decades.

Beyond his tours and recordings, Mr. Bolling was a prolific composer of scores for French TV and films such as “Borsalino” (1970), a gangster movie set in 1930s Marseille and starring Jean-Paul Belmondo and Alain Delon, and “Le Magnifique” (1973), a sendup of espionage sagas featuring Belmondo and Jacqueline Bisset. In the pop genre, Mr. Bolling arranged music for Brigitte Bardot and Juliette Gréco, among other French stars of the day.

In the United States, he was best known for his crossover compositions, which he recorded with classical musicians including flutist Jean-Pierre Rampal, cellist Yo-Yo Ma and pianist Emanuel Ax. Writing in the New York Times in 1982, music critic Allan Kozinn described Mr. Bolling as “the leader of the pack in the crossover world.”

Many musical purists looked down on the genre, disdaining renditions of Bach on the synthesizer and performances by Luciano Pavarotti in which the Italian tenor sweatily belted out Neapolitan standards. Mr. Bolling distinguished himself, Kozinn observed, because “rather than tamper with the standards,” he created works of his own.

“Mr. Bolling’s compositional strategy involves giving his classical soloist a through-composed part, written in a style replete with Baroque and classical gestures and allusions to the featured instrument’s repertory and idiomatic uses,” Kozinn wrote. Meanwhile, “his own piano, bass and percussion trio interacts with a lightweight jazz counterpoint. It is a formula that seems to have been consistently successful.”

Mr. Bolling’s most famous crossover number was his “Suite for Flute and Jazz Piano,” recorded with Rampal on a 1975 album that was nominated for a Grammy for best chamber music performance. The album became one of the top-selling classical recordings of all time, landing on the Billboard charts and staying there for 10 years.

Mr. Bolling received Grammy nominations in 1979 for his album “Suite for Violin and Jazz Piano,” recorded with violinist Pinchas Zukerman, and in 1987 for “Suite No. 2 for Flute and Jazz Piano Trio,” again with Rampal.

His melding of jazz and classical music succeeded, he said, because he kept to his lane and allowed his classical collaborators to stay in theirs.

“There are no syncopations, no blue-notes and no jazz feeling in their parts,” he told Kozinn. “Those things are reserved for the piano, bass and drums. Have I ever been tempted to stretch their parts into jazz? No. Why should I? It would spoil the contrast.

“As classical players, these musicians are so good, and they handle melody and expression so beautifully,” he continued. “Yet, they rarely have the opportunity to do that with new pieces, because contemporary classical music is — well, you know what it is. So, although I began writing these pieces just for fun, I now realize that they serve another purpose. They allow these performers to add new material in a classical style to their repertories, music with plenty of melody and, because of the jazz element, a new kind of sound.”

Claude Bolling was born in Cannes, on the French Riviera, on April 10, 1930. His father was a hotel manager.

Except for a wartime sojourn in Nice, Mr. Bolling spent much of his upbringing in Paris, where he said he gained an appreciation of jazz at nightclubs and music halls. He told the Jerusalem Post that by the time he developed an interest in classical music, he was too old to enroll at the conservatory and therefore pursued private lessons.

In his teens, he won a prize from the Hot Club de France, an organization founded to promote jazz in France. According to his website, he formed his first orchestra at 16 and was 18 when he made his first record.

In the early years of his career, he performed with visiting American jazz musicians including cornetist Rex Stewart, trumpeter Buck Clayton and vibraphonist Lionel Hampton. Such was his allegiance to Ellington that the French writer Boris Vian dubbed Mr. Bolling “Bollington.”

He first experimented with crossover music in 1970, when he was invited to perform an original composition on television. He proposed a four-hands performance with a friend of his, the classical pianist Jean-Bernard Pommier, and the two concluded that the combination of their talents worked.

The Guardian reported that Mr. Bolling’s wife, Irène Dervize-Sadyker, died in 2017 after nearly six decades of marriage, and that his survivors include two sons, David and Alexandre.

Mr. Bolling jokingly described some classical music disciples as “more royalist than the king when it comes to claiming a superiority over jazz.” For his own part, he was little concerned with the labels others put on his opus.

“What people say the music is, or is not, does not disturb me,” he told Kozinn. “I’m not writing pieces that are very concerned with . . . presenting the image of genius. I am only writing music for fun. I try to maintain a certain level of taste and quality, of course. But my main purpose is to make the musicians happy, and if possible, to make the audience happy too.”