Kurt Thomas, an electrifying gymnast whose scissor-kicking maneuvers sent crowds into a frenzy and helped him become the first American man to win a world championship, only to see his quest for Olympic gold thwarted by the U.S. boycott of the 1980 Games in Moscow, died June 5. He was 64.

His death was confirmed by USA Gymnastics and first reported by International Gymnast Media, which said he had a stroke on May 24. Additional details were not immediately available.

When Mr. Thomas won his first world championship in 1978, he was arguably the greatest male gymnast the United States had yet produced. Known for his “Thomas flair,” a widely imitated series of midair scissor kicks on the pommel horse, he burst onto the American sports scene as a gymnast of uncommon creativity and unprecedented fame.

Interviewed by Johnny Carson and Dick Cavett, he became the first gymnast to win the James E. Sullivan Award as the nation’s top amateur athlete of 1979. En route to victory at the American Cup gymnastics competition that same year, he drew such thunderous applause at Madison Square Garden that a television commentator likened him to a young Frank Sinatra performing at the Paramount Theatre.

“He is redefining the sport of men’s gymnastics, putting it on a par with the women’s version as a crowd-pleasing act,” wrote New York Times journalist Grace Lichtenstein. “Like John Curry, the ice skater,” she added in a 1979 profile, “he is taking a sport and grafting on new elements to make it an art. He is men’s gymnastics’ Baryshnikov and its Balanchine.”

Mr. Thomas was brash and cocky — “the John McEnroe of gymnastics,” U.S. Gymnastics chief Mike Jacki once said, referring to the confrontational tennis star — and stood just 5-foot-5, with long arms and short legs. He was so undersized as a child that his mother took him to a geneticist when he was 9, fearing that he would never grow. “He was so tiny that if he had lost five pounds I think he would have died,” she told Sports Illustrated in 1978. Mr. Thomas filled out just enough to become a force on the gymnastics floor, taking up the sport as a teenager in Miami.

At Indiana State University in Terre Haute, he won five individual NCAA titles and led the Sycamores to the 1977 national championship, while ranking behind future basketball Hall of Famer Larry Bird as the school’s second-biggest celebrity. “Pure Gold in the Corn Belt,” Time magazine proclaimed in a dual profile of the athletes.

In 1978, Mr. Thomas traveled to Strasbourg, France, for the World Artistic Gymnastics Championships, where he helped break a nearly five-decade drought for American gymnasts. Not since the 1932 Olympics had an American won at a major international competition, and suddenly the country had two gold medalists, with Mr. Thomas winning for his floor exercise and Marcia Frederick winning on the uneven bars.

Mr. Thomas successfully defended his floor title at the next year’s world championships in Fort Worth. He took home six medals — including two golds, for the floor exercise and horizontal bar — and placed second in the all-around competition, behind only Soviet gymnast Alexander Dityatin.

“It’s time for the world to look out for American gymnasts. We’ve arrived,” Mr. Thomas told reporters, looking ahead to the 1980 Moscow Games. He had competed at the Montreal Olympics four years earlier but finished 21st in the all-around, while nursing a finger injury and still honing his technique.

In the lead-up to Moscow, he said, he felt unbeatable. “In my mind and my heart,” Mr. Thomas later told Florida’s Sun-Sentinel newspaper, “I knew I was the best at that time.”

But when President Jimmy Carter announced that the United States would boycott the Games in response to the Soviet invasion of Afghanistan, Mr. Thomas decided to turn pro. With his Olympic eligibility no longer an issue, he gave up his amateur status to take advantage of lucrative endorsement offers, tour the country with a gymnastics group and perform at Sea World and state fairs.

Mr. Thomas also wrote an autobiography, coached gymnastics, launched a line of activewear and began a short-lived acting career, starring in the action movie “Gymkata” (1985), directed by “Enter the Dragon” filmmaker Robert Clouse, as a gymnast who enters a deadly competition and uses his acrobatic abilities to fight off a crowd in the “Village of the Crazies.”

After working as a television commentator during the 1984 Olympics, Mr. Thomas staged an unexpected comeback attempt for the 1992 Games. He reached the U.S. Olympic trials at age 36, more than a decade older than most of his competitors, but failed to make the team.

By then, some news stories had mistakenly referred to him as a “former Olympic gold medal gymnast.” Others noted that by the time he was 31, he was twice divorced and bankrupt, running into problems with his spending and with “high-risk investments” suggested by one of his managers.

But Mr. Thomas insisted he was not bitter. “Look at it this way,” he told the Sun-Sentinel in 1989. “I could have gone to the Olympics and placed 50th. What if I had gone over there and lost? I think things would have been real bad.”

Even if he had been offered commercials for Campbell’s soup and Kentucky Fried Chicken, as he had without going to the Games, “I still would have lost the money,” he continued. “I still would have been stupid. I’d still have to grow up.”

The way things panned out, he added with some surprise, “I’m a hero without proving it, a gold medalist without ever doing it.”

Kurt Bilteaux Thomas was born in Miami on March 29, 1956. His father was a meat-company manager and former wrestler who died in a car accident when Kurt was 7, leaving his mother, a secretary, with four children to raise.

At 14, he saw a junior college gymnastics team practice and became entranced by their horizontal bar routines. He was 4-foot-9 and 77 pounds when he began competing as a high school freshman, and by his senior year, he was fielding a scholarship offer from Indiana State gymnastics coach Roger Counsil, who later led the men’s national team.

“I had never had anybody tell me I was good at anything before,” Mr. Thomas told the Philadelphia Daily News in 1981. “Gymnastics made me feel like somebody.”

While his gymnastics career began to take off, Mr. Thomas — who received a bachelor’s degree in 1979 — married fellow Indiana State student Beth Osting and moved into an 8-foot-wide trailer outside Terre Haute, extending his training regimen to six hours a day, six days a week. Their marriage later ended in divorce, as did a subsequent marriage to gymnast Leanne Hartsgrove.

In 1996, he married Beckie Jones, a dancer and choreographer with whom he opened Kurt Thomas Gymnastics in Frisco, Tex. Complete information on survivors was not immediately available.

Mr. Thomas was inducted into the USA Gymnastics Hall of Fame in 1990 and the International Gymnastics Hall of Fame in 2003.

“To really hit your routine the best you can possibly hit it, that’s just a super feeling,” he told the Times during his championship run in 1979.

He had a similar feeling at the world championships later that year, when the national anthem was played and the American flag raised in recognition of the gold medals won by Mr. Thomas and fellow American Bart Conner, who won the gold for parallel bars.

“Our flag has been down at the bottom of the box so long,” Mr. Thomas said, “that it was great to see it flying at the top today.”