Toots Thielemans in 2009. (Rob Verhorst/Redferns via Getty Images)

Jean-Baptiste “Toots” Thielemans, whose unrivaled mastery of the harmonica led to collaborations with jazz luminaries such as pianist Oscar Peterson and trumpeter Dizzy Gillespie, and whose mellifluous sound graced soundtracks as varied as “Midnight Cowboy” (1969) and the television show “Sesame Street,” died Aug. 22 in Belgium. He was 94.

The cause was complications from a fall, his agent, Veerle Van de Poel, told Belgian media.

Unlike the smaller, diatonic harmonica favored by blues musicians, Mr. Thielemans performed on the slightly larger chromatic harmonica, which has a spring-loaded slide that allows the performer to play every note in every key — sharps and flats included — on a three-octave scale.

His sound was at once wistful and wiry, full of complicated runs and languid tones. It was a sound, he often said, that was “in that little space between a smile and a tear.”

Mr. Thielemans was also an accomplished guitarist and a first-rate jazz whistler — he often played his most famous composition, “Bluesette,” on the guitar while whistling the melody one octave higher. The Beatles’ John Lennon reportedly switched to a solid-body Rickenbacker 325 guitar after seeing Mr. Thielemans play the instrument during a tour of Germany with the George Shearing Quintet, then one of the hottest jazz groups in the United States, with which he played from 1952 to 1959.

"Toots" Thielemans in 2007. (JOCK FISTICK/BLOOMBERG NEWS PHOTO / JOCK FIST)

But it was Mr. Thielemans’s inimitable technique and distinctive sound on the chromatic harmonica, an instrument that he single-handedly proved could hold its own in the jazz repertory, for which he will be remembered.

“He has a level of virtuosity that you don’t have to make excuses for, you don’t have to put an asterisk on Toots. . . . You don’t have to say, ‘He’s great — for a harmonica player,’ ” said jazz critic Gary Giddins, who spoke as part of a 2006 New York University jazz master class with Mr. Thielemans. “He can sit up there with Dizzy and doesn’t have to take an apology because of the instrument. That’s the genius of the whole thing.”

Indeed, it was Mr. Thielemans’s then-novel arrangement of “Stardust,” the 1927 jazz standard by Hoagy Carmichael, cut on an acetate disc in a garage studio in his home town of Brussels, that caught the attention of American bandleader Benny Goodman. Mr. Thielemans played guitar in Goodman’s sextet for a European tour in 1950, but he also persuaded the leader to allow him at least one number on harmonica each night. Not long after, he settled permanently in the United States.

Although Mr. Thielemans never shied away from commercial work — for TV, film and commercials, such as his whistling jingle in televised Old Spice deodorant ads — he was revered by many musicians for his unerring sense of harmony and his quicksilver runs, cementing his jazz harmonica reputation with a 1979 duo album, “Affinity,” with pianist Bill Evans.

Two years later, after having a stroke just shy of his 60th birthday, Mr. Thielemans lost most of the strength in his left hand, which cut his guitar playing down to just one or two numbers per show but also brought a new profundity to his phrasing.

“I can’t play bebop now,” he later told Dutch journalist René Steenhorst. But, he added, “All those fast notes didn’t make me that popular anyway. It’s much better now. I actually play the main line of a tune now. I say more with less.”

Jean-Baptiste Frédéric Isidor Thielemans was born April 29, 1922, in the still-gritty, working-class Brussels neighborhood known as the Marolles. His parents ran a cafe on the same street where Pieter Bruegel the Elder lived in the 16th century.

Self-taught on accordion as a child, Mr. Thielemans bought his first harmonica while he was a teenager in Brussels after watching an American movie in which a prisoner was playing the harmonica while awaiting the electric chair. “That’s nice,” he remembered thinking, according to a 2011 interview with a Smithsonian jazz project. “I’m going to buy one.”

The guitar, he told the jazz magazine DownBeat, was acquired by chance when he was 21, during the German occupation of Belgium. Mr. Thielemans was home in bed with a lung infection, and a musician friend stopped by with a black-market guitar, expressing frustration with his inability to perfect a certain jazz lick.

“We were listening to Fats Waller records like ‘Hold Tight’ — there’s the quintessence of the jazz scale and everything you need in the blues in that song,” he explained. “I knew the song, but I’d never touched a guitar. I said that if he’d give me five minutes, I’d play ‘Hold Tight’ on one string. I played it, and he gave me the guitar.”

Mr. Thielemans had been enrolled as a math major at the Université Libre de Bruxelles, but playing music became increasingly important, and he never graduated.

When the war ended, Mr. Thielemans played jazz at the many officer’s clubs that sprung up in Belgium after the liberation. Records called “V” or “Victory” discs were widely available as well, and Mr. Thielemans was able to listen to recordings by American jazz stars such as Louis Armstrong and Art Tatum.

Mr. Thielemans also started studying the work of bebop saxophonist Charlie Parker, and took his nickname, according to one version of the story, from sax player Toots Mondello and trumpeter Toots Camarata. “Toots,” he liked to quip, “sounds a lot more hip than Jean-Baptiste.”

In 1947, when he was 25 years old, Mr. Thielemans went to visit his uncle in Miami, and at a jam session there, met Bill Gottlieb, whose seminal jazz photos of the 1940s put him on a first-name basis with the music world’s biggest stars.

When Mr. Thielemans arrived in New York, Gottlieb took him to a nightclub on 52nd Street and introduced him to the members of a group fronted by Howard McGhee, a trumpeter and composer who was at the forefront of modern jazz. By then, he had already played with Parker at a Paris jazz festival in 1949 and jammed with him in Sweden.

As Mr. Thielemans recalled in a 2006 DownBeat magazine interview, he took out his harmonica and began playing. “In those days, the big identity, the key to the bebop door, was the third and the fourth bar of ‘I Can’t Get Started,’ ” he said. “I played it [and] the whole band fell on the floor. I was in after two measures.”

From there, Mr. Thielemans became a member of the Shearing Quintet — “the only permanent job I ever had” — he has said, before embarking on a successful freelance career as a guitarist, and, increasingly, a chromatic harmonica player.

In addition to his ample studio session work, he also recorded or toured with Quincy Jones, Dinah Washington, Paul Simon and a bevy of other singers and musicians. Furthermore, the lyrics of Mr. Thielemans’s composition “Bluesette” were penned by “Girl From Ipanema” lyricist Norman Gimbel, and it was recorded to great acclaim by Sarah Vaughan in 1964.

Mr. Thielemans became an American citizen in 1957, which, at the time, meant the mandatory loss of his Belgian citizenship. But it was regained in 2001, when Belgian King Albert II named the musician a baron.

Mr. Thielemans’s first wife, Netty de Greef, died of cancer in 1977. A few years later, he married Huguette Tuytschaever, a Belgian artist. Survivors could not be immediately determined.

Mr. Thielemans made his final recording in 2012 during his 90th birthday tour and gave his last performance in 2014, when he played at the Jazz Middelheim festival in Antwerp, Belgium.

“It’s hard, it’s almost a crazy instrument to want to play,” Mr. Thielemans once told an interviewer about the chromatic harmonica. Yet he was preeminently a jazz musician; and it was the notes, not the instrument, that counted — his harmonica, he liked to say, “could be a broomstick or a tuba.”