Her manager, Clifton Henderson, said she had been ill for some time, but he released no cause of death.
Ms. Simone had only one Top 20 hit in her long career — her very first single, "I Loves You, Porgy," released in 1959 — but her following was large and loyal and her impact deep and lasting. Aretha Franklin, Roberta Flack and Laura Nyro were among the singers who were influenced by her. In recent years her songs resurfaced and won new fans on television commercials and in dance-club remixes.
Although she was most often characterized as a jazz singer, Ms. Simone, who usually performed with a rhythm section and always accompanied herself on piano, was almost impossible to classify.
"If I had to be called something," she wrote in 1991 in her autobiography, "I Put a Spell on You," "it should have been a folk singer because there was more folk and blues than jazz in my playing."
But her piano playing also revealed her classical training more clearly than most jazz pianists', and her singing — at times rough and raw, at other times sweet and pure — owed an unmistakable debt to black gospel music. Her repertory was similarly eclectic: it ranged from blues to Broadway, from Jacques Brel to Screamin' Jay Hawkins to the Bee Gees.
Ms. Simone was as famous for her social consciousness as she was for her music. In the 1960's no musical performer was more closely identified with the civil rights movement. Though she was best known as an interpreter of other people's music, she eloquently expressed her feelings about racism and black pride in those years in a number of memorable songs she wrote herself.
"Mississippi Goddam" was an angry response to the killing of the civil rights advocate Medgar Evers. "Young, Gifted and Black," written with the keyboardist Weldon Irvine Jr., became something of an anthem, recorded by Aretha Franklin and many others. "Four Women" painted a subtle but stinging picture of the suffering and the strength of African-American women.
She was born Eunice Waymon on Feb. 21, 1933, in Tryon, N.C., and grew up singing in a church choir and studying piano. She received a scholarship to the Juilliard School of Music in 1950, although she had to work as an accompanist for singers and as a piano teacher to help support herself. She eventually ran out of money, left Juilliard and moved back in with her family, at that time living in Philadelphia.
In 1954 she got a job playing piano at a bar and grill in Atlantic City, where she assumed her stage name — because, she later explained, she did not want her mother to find out what she was doing. After her first night on the job, she was told that she had to sing as well as play, so she began emulating Billie Holiday and other singers she admired. She later said that she kept herself from getting frustrated with the often-indifferent crowds by playing the piano in a manner "as close to classical music as possible." This unusual mixture of approaches produced what the music writer Ashley Kahn has called "an impassioned, impromptu approach that became her signature."
Ms. Simone soon began to work in better venues and develop a devoted following. In 1958 she signed with Bethlehem Records; a few months later, she was on the pop charts. One of her best-remembered hits was "My Baby Just Cares for Me."
Her subsequent recordings for the Colpix, Philips and RCA Victor labels established her as a potent attraction on the cabaret, concert and festival circuits. Unafraid to speak her mind, she frequently clashed with promoters and occasionally berated her audiences for not paying attention, but her temperament did nothing to diminish her appeal.
Her survivors include three brothers, a sister and a daughter, Lisa, a singer and actress known professionally as Simone who is currently appearing on Broadway in "Aida."
In the 1970's her music fell out of fashion in the United States; she divorced her husband and manager, Andy Stroud, and beset by financial problems she left the country in 1973, living in Liberia and Barbados before settling in France. In a 1998 interview, she said she had left the United States because of a racial situation she called "worse than ever."
In recent years, as her health began to fail, Ms. Simone performed less and less, although she continued to draw enthusiastic crowds wherever she appeared. Al Schackman, who played guitar in her backup group for four decades, said she had recently canceled a tour of Britain but had been planning a United States tour for this spring.