Dennis “Dee Tee” Thomas, a saxophonist and a founding member of the band Kool & the Gang, died on Saturday in New Jersey. He was 70.

Mr. Thomas died in his sleep, according to a statement from his representatives that did not specify a cause of his death or where in New Jersey he died.

Mr. Thomas was a co-founder of the long-running band Kool & the Gang, known for hits such as “Celebration,” “Get Down on It” and “Jungle Boogie.” He saw the band, which experimented with sounds from soul, funk, jazz, pop and R&B, through numerous lineup changes.

Mr. Thomas was a “huge personality” in the band, his representatives said, and he helped style the performers’ wardrobes to ensure “they always looked fresh.”

“Dennis was known as the quintessential cool cat in the group, loved for his hip clothes and hats, and his laid-back demeanor,” the statement said.

The band won a Grammy Award in 1978, the decade when several of its upbeat hits climbed the charts.

Around the time the band won a Grammy, it entered a slow period before adding a new vocalist, J.T. Taylor, and adapting its sound to match the disco sensibilities of the era. The group re-emerged in 1979 with the smash “Ladies’ Night,” an ode to a night of partying and dancing.

The band members followed the hit with the 1980 song “Celebration,” a timeless classic that embodied the group’s buoyant sound. The track became a staple at sporting events and any other displays of joy and enthusiasm. The song was inducted into the National Recording Registry of the Library of Congress, an honor reserved for 25 songs every year that showcase the rich heritage of American music.

The band members lent their voices to the 1984 charity single “Do They Know It’s Christmas?” A number of the decade’s biggest artists recorded the track to draw attention to a famine in Ethiopia.

Mr. Thomas formed Kool & the Gang in 1964 with six of his friends — Robert Bell, known as Kool; Ronald Bell; Spike Mickens; Ricky Westfield; George Brown; and Charles Smith. They first called themselves the “Jazziacs,” the statement said, before settling on the name “Kool & the Gang,” a nod to Robert Bell.

“We learned that we had to simplify, that most simple music will grab a wide part of the audience,” Mr. Thomas told The New York Times in 1973, about choosing the group’s musical style. “Everybody in the group was a jazz musician at heart, but we knew we had to play R&B to make money.”

Mr. Thomas was the band’s “budget hawk” in the early days, his representatives said, adding that he could be seen “carrying the group’s earnings in a paper bag in the bell of his horn.”

Mr. Thomas’s alto saxophone solos were featured on several of the band’s tracks. He could also play the flute and percussion instruments, and he was the master of ceremonies at the band’s shows.

His last performance with the band was on July 4 at the Hollywood Bowl in Los Angeles.

Dennis Thomas was born on Feb. 9, 1951, in Orlando, Fla. He and his parents moved to Jersey City, N.J., when he was 2 years old, The Times reported in 1973. He grew up in the city’s Lafayette section, where he met the other founding members of Kool & the Gang.

“We want to play a universal music,” Mr. Thomas said in 1973. “We want to lift our audiences up so they think about what they’ve heard.”

The band had a dozen top 10 hits on the Billboard charts, and the group received a star on the Hollywood Walk of Fame in 2015.

Mr. Thomas was married to Phynjuar Saunders Thomas and lived in Montclair, N.J., his representatives said.

One of his daughters, Michelle Thomas, was an actress on television shows, including “The Cosby Show,” “Family Matters” and “The Young and the Restless.” She died in 1998 of cancer at age 30. He was also preceded in death by another of his daughters, Tracy Jackson.

In addition to his wife, he is survived by his daughter Tuesday Rankin; his sons David Thomas and Devin Thomas; his sisters Doris Mai McClary and Elizabeth Thomas Ross; his brother Bill Mcleary; an aunt and several nieces, nephews and grandchildren, the statement said.