Dennis Thomas performs in 2016 at the Jazz in the Gardens Music Festival in Florida. (Mychal Watts/Getty Images for Jazz in the Gardens)

Dennis “Dee Tee” Thomas, a saxophonist who co-founded Kool & the Gang and remained one of its core players for more than five decades, propelling hits like “Jungle Boogie” and “Celebration” with his funky, euphoric horn blasts, died Aug. 7 in New Jersey. He was 70.

The band announced his death in a statement but did not say exactly where or how he died. He had been living in Montclair, N.J., and last performed with the group at a Fourth of July concert this year at the Hollywood Bowl in Los Angeles.

In addition to playing the alto sax, Mr. Thomas was a flutist and percussionist for Kool & the Gang, which was formed in Jersey City in 1964 as a jazz combo called the Jazziacs. Stretching out to incorporate soul, funk and R&B, the band recorded buoyant dance floor staples such as “Hollywood Swinging” and “Jungle Boogie,” and then got a lead singer — James “J.T.” Taylor — and developed a lighter, more pop-oriented sound.

That evolution helped make Kool & the Gang one of the biggest bands of the 1980s, when it climbed the Billboard Hot 100 with songs such as “Ladies’ Night” (“Oh, what a night”) and the chart-topping “Celebration,” a horn-driven fixture of wedding receptions, sporting events and family cookouts. In March, the song was selected for the Library of Congress’s National Recording Registry.

“We can pretty much tell when a song will be a hit,” Mr. Thomas told the Detroit Free Press in 1987. “When it’s finished it has a certain spark. . . . The big surprise to me was ‘Celebration.’ I thought it would do well, but not that well. I remember struggling with the song and saying, ‘I think we’re finished with this one.’ ”

Starting out in Jersey City, Kool & the Gang had little interest in pop or R&B, preferring to gather around a record player at trumpeter Spike Mickens’s home to listen to jazz albums by John Coltrane, Wes Montgomery and Paul Chambers. “We were anti-Motown. Mm-mm, nope,” Mr. Thomas said in a video interview that the band released in February.

The original seven-member lineup were brothers Robert “Kool” Bell and Ronald Bell, who later went by Khalis Bayyan, and their teenage friends Mr. Thomas, Mickens, Ricky Westfield, George Brown and Charles Smith. After five years and several name changes, they became known as Kool & the Gang in 1969, placing Kool’s name front and center but maintaining a collaborative approach to running the band.

“We came to realize what kind of people we were playing for then,” Mr. Thomas told the New York Times in 1973, looking back on the group’s turn toward R&B. “They weren’t developed music lovers. They didn’t understand the difficulty of some of the music being played. And the youngsters didn’t dig jazz then. We learned that we had to simplify, that most simple music will grab a wide part of the audience.”

While Kool & the Gang often celebrated joy itself — “Celebrate good times, come on,” the band sang in “Celebration” — the group also addressed social issues, as in “Who’s Gonna Take the Weight,” from their 1971 album “Live at the Sex Machine.” Mr. Thomas came up with the song’s prologue, writing spoken-word lyrics evoking the political violence and environmental activism of that era:

“We’re gonna have to learn to live together and love each other. Because I believe one day, someone or something is gonna wanna judge who’s creating all this corruption and death and pollution and all these difficult situations on Earth. And he’s gonna wanna know: Who’s gonna take the weight?”

Speaking for the band, Mr. Thomas told the Times, “We want to play a universal music. We want to lift our audiences up so they think about what they’ve heard.”

Dennis Ronald Thomas was born in Orlando on Feb. 9, 1951. He was 2 when he moved to Jersey City with his family, and 18 when Kool & the Gang released its self-titled debut album in 1969. By then, the group had spent several years playing in local clubs, where they were sometimes paid with sandwiches and potato chips.

The band recorded a dozen Top 10 hits in all, including the 1980s pop ballads “Joanna” and “Cherish,” and reached a new generation of listeners in the 1990s, when their songs were used in movies such as “Pulp Fiction” and sampled by hip-hop acts including Nas, Luniz, Eric B. & Rakim, N.W.A. and Mase.

“It’s absolutely unspoken how his saxophone has breathed life into so many songs in hip hop,” the musician and producer Questlove wrote in an Instagram tribute to Mr. Thomas after his death. “Let alone soul/funk/jazz/disco.”

Survivors include his wife, Phynjuar Saunders Thomas; three children, Tuesday Rankin, David Thomas and Devin Thomas; two sisters; a brother; and many grandchildren. His daughter Michelle Thomas, an actress who appeared on sitcoms including “The Cosby Show” and “Family Matters,” died of cancer at age 30 in 1998. He was also predeceased by another daughter, Tracy Jackson.

Kool & the Gang continued to record into the 2010s, while touring with acts including the Dave Matthews Band and Van Halen. With the deaths of Mr. Thomas and of Ronald Bell last year, only bassist Kool Bell and drummer Brown survive from the group’s original lineup.

In its statement, the band called Mr. Thomas “the quintessential cool cat in the group,” saying that he “was the group’s wardrobe stylist who made sure they always looked fresh.” For a time, the statement said, he also played a dual leadership role, serving as master of ceremonies at shows and as the “budget hawk” backstage, “carrying the group’s earnings in a paper bag in the bell of his horn.”