Adolph Kiefer, 1936 Olympic champion swimmer. (Courtesy of family of Adolph Kiefer/Courtesy of family of Adolph Kiefer)

Adolph Kiefer, the oldest living U.S. Olympic champion in any sport, whose dominance in the backstroke was unrivaled in the 1930s and 1940s and who was the Navy’s chief swimming instructor during World War II, died May 5 at his home in Wadsworth, Ill. He was 98.

The cause was a heart attack, said a grandson, Robin Spencer Kiefer.

Mr. Kiefer set his first world record as a 16-year-old Chicago schoolboy in 1935 and easily won the 100-meter backstroke at the 1936 Olympics in Berlin. Swimming in an outdoor pool under a driving rain while wearing a full-body wool swimsuit, he set an Olympic record that would not be broken for 20 years.

“No one who saw today’s race could honestly deny that he had witnessed the greatest back-stroker that has ever swum,” wrote New York Times correspondent Albion Ross, who was covering the Berlin Games. “There probably are a dozen athletes of all sorts who are in a class by themselves where they need fear no competition and are beyond comparison with rivals. To this exclusive, almost supernaturally gifted company Kiefer belongs.”

From 1934 until his retirement from competitive swimming in 1946, Mr. Kiefer would lose only two times in more than 2,000 races. He was the first person to swim the 100-yard backstroke under 1 minute.

Olympic gold medalist Adolph Kiefer in 1937. (Chicago Tribune archive photo/Acme photo)

He set at least 17 world records at distances ranging from 100 yards to 1,500 meters and was a national champion no fewer than 28 times, including in freestyle and individual medley events.

Mr. Kiefer was inducted into the International Swimming Hall of Fame in its inaugural year of 1965 and is regarded as one of the greatest backstroke swimmers in history. Yet because the Summer Olympics were canceled in 1940 and 1944, when Mr. Kiefer was at the peak of his abilities, his fame never quite matched his achievements.

At his only Olympic competition in 1936, Mr. Kiefer — whose parents were German immigrants — was introduced to Nazi leader Adolf Hitler, who reportedly called the strapping young swimmer “the perfect example of the true Aryan.”

“I remember him being a small man with a small hand,” Mr. Kiefer recalled of the meeting in a 2000 interview with the New York Times, “and his handshake wasn’t a firm one.”

Hitler viewed the Olympic Games as an international showcase of German might. What he didn’t know was that one of Mr. Kiefer’s closest friends on the U.S. team was African American track star Jesse Owens. When Owens won four gold medals, the world recognized his victories as a repudiation of Hitler’s notions of Aryan racial superiority.

Mr. Kiefer, who remained friends with Owens until the latter’s death in 1980, came to detest his first name of Adolph.

“I’ve heard him say a hundred times,” Robin Kiefer said of his grandfather’s encounter with Hitler, “ ‘If I had known then what I know now, I would have thrown him in the pool and drowned him.’ ”

Olympic gold medalist Adolph Kiefer with water sports items from his business in 1961. (UPI photo/UPI)

During World War II, Mr. Kiefer enlisted in the Navy. He was shocked to learn that many naval officers didn’t know how to swim and that troops on ships were more likely to die by drowning than from enemy attack.

Mr. Kiefer was put in charge of a Navy-wide swimming safety program, training thousands of instructors, who in turn taught water survival techniques to millions of sailors. He devised the “victory backstroke,” which was credited with saving thousands of lives after shipwrecks and naval battles.

“The biggest thing I did in my life was not the gold medal,” Mr. Kiefer told the Chicago Daily Herald in 2016. “It was going into the Navy and saving lives.”

Adolph Gustav Kiefer was born June 27, 1918, in Chicago. His father was a candy maker who died when his son was 11.

Mr. Kiefer’s introduction to swimming came by accident, when he fell into a canal.

“I learned to swim in a Chicago drainage ditch,” he told the Chicago Tribune in 2014. “I didn’t know how to swim. But I floated on my back and kicked my feet.”

He was drawn to the backstroke because he didn’t like to inhale water through his nose, and he showed such promise that he commuted to the University of Michigan to train with a college swimming champion.

Mr. Kiefer set his first world record, at 150 yards, in April 1935. Three months later, he swam the 100-yard backstroke in 59.8 seconds, the first time anyone had recorded under a minute. He continued to lower the record in the coming years, with a personal best of 56.1 seconds in 1944. He also introduced a flip turn still used by backstroke swimmers.

He attended the University of Texas, Northwestern and Columbia before entering the Navy.

After retiring from competition in 1946, Mr. Kiefer was said to be under consideration for the role of Tarzan, made famous by Johnny Weissmuller, a swimming champion of the 1920s. Instead, he went into business, founding a company that made swimming and diving equipment.

Mr. Kiefer, who had 14 patents, manufactured the first nylon swimsuits and invented rescue devices and the kickboard, an item universally used in training and to teach young swimmers. He also invented the first turbulence-reducing lane divider, which limited the amount of choppy water flowing from one lane to the next, creating a smoother pool surface. He continued to run his business well into his 90s.

Mr. Kiefer served on presidential physical fitness commissions and helped lead efforts to teach swimming to children.

Even after neuropathy caused him to lose the feeling in his feet, he continued to swim every day until shortly before his death.

His wife of 73 years, the former Joyce Kainer, died in 2015. Survivors include four children, Dale Kiefer of Pacific Palisades, Calif., Jack Kiefer of Wadsworth, Kathy Priest of New Haven, Conn., and Gail Lucks of Denver; 14 grandchildren; and 16 great-grandchildren.

When Mr. Kiefer was named to the 1936 U.S. Olympic team, he had just graduated from high school. He had to hitchhike to New York, his grandson said, to board the Olympians’ ship for Europe.

In recent decades, Mr. Kiefer came to lament the growing professionalism of the Olympics. He preferred the code of the amateur, in which unpaid athletes competed strictly for the love of sport.

“The Olympics should be athletes from different countries getting to know each other,” he said in 2000. “There has to be a way to return to a purer form of amateurism, to sports for the sake of promoting fitness and health — not just money.”