Debbie Friedman, a singer and songwriter whose work — which married traditional Jewish texts to contemporary folk-infused melodies — is credited with helping give ancient liturgy broad appeal to late-20th-century worshippers, died on Sunday in Mission Viejo, Calif. She was 59 and lived in Laguna Woods, Calif.

The cause was complications of pneumonia, said Jerry Kaye, a family spokesman. Ms. Friedman, who continued performing in public until the end of her life, had been ill for the past two decades with a chronic, often debilitating and never definitively diagnosed neurological condition.

One of the brightest stars of the Jewish music world, Ms. Friedman was called “the Joan Baez of Jewish song,” as the Jewish newspaper The Forward wrote in 1995. She was known for her clear, strong voice and for the intense spiritual conviction with which she sang as she accompanied herself on the guitar.

She recorded more than 20 albums, which together have sold half a million copies. Among them are “Miracles & Wonders,” “Renewal of Spirit,” “You Shall Be a Blessing” and “The Water in the Well.”

Ms. Friedman’s compositions encompass not only modern settings of traditional Hebrew liturgy but also songs for which she wrote original English lyrics. Regularly sung by congregants in Reform, Reconstructionist, Conservative and some Modern Orthodox synagogues (as well as in some Christian churches), they are widely credited with having revitalized worship for a generation of postwar American Jews.

To an extent, her work also made its way into the mainstream marketplace. Her music appears on the video “Barney in Concert,” on which the purple dinosaur sings her setting of the Hebrew alphabet for children; her lyrics have been featured on a line of Hallmark cards. In live performance, Ms. Friedman sang on some of the world’s most storied concert stages, including Carnegie Hall.

In 2007, Ms. Friedman joined the faculty of the School of Sacred Music at Hebrew Union College-Jewish Institute of Religion in New York, where she taught Reform rabbinical and cantorial students; she later taught at Hebrew Union College’s Los Angeles campus.

Her appointment was striking for two reasons: first, because she was a largely self-taught musician who did not know how to read music, and second, because her work — inclusive, progressive and strongly feminist — was perceived as a threat to established cantorial tradition when she began her career in the early 1970s.

Deborah Lynn Friedman was born on Feb. 23, 1951, in Utica, N.Y., to parents who belonged variously to Conservative and Reform synagogues. When she was a child, the family moved to Minnesota, and she grew up in St. Paul.

As a teenager, she was enraptured both by Jewish and folk music; she taught herself to play the guitar from the records of Peter, Paul and Mary, and her music would be likened to theirs.

After high school, Ms. Friedman worked briefly on an Israeli kibbutz before returning to the United States.

“One night I went to synagogue, and realized, sitting there, I was bored,” she told The Los Angeles Times in 1995. “I realized the rabbi was talking, the choir was singing and nobody was doing anything. There was no participation.”

Not long afterward, an original melody came to her, and as an experiment, she set to it the words of “V’ahavta,” a prayer drawn from Deuteronomy that commands Jews to love God.

“I sang it with some high school kids, who sang it arm in arm, crying and singing,” Ms. Friedman told Lilith magazine in 1995. “They were looking for a spiritual avenue of their own.”

With that, Ms. Friedman had found her calling; her first album, “Sing Unto God,” a collection of Sabbath songs, was released in 1972.

While some rabbis and cantors welcomed her music as a democratizing force, others saw it as a subversive breach of time-honored tradition, in which the cantor was typically white-haired, always male and usually vocally imposing and the congregants were passive listeners.

By contrast, Ms. Friedman’s music emphasized audience participation. (At her concerts, she encouraged audience members to sing along; many also danced in the aisles.) It centered on themes like healing, a concern that stemmed partly from her years of chronic illness. (Her most famous song is a setting of “Mi Shebeirach,” a Hebrew prayer for the sick.)

Many of her English lyrics concerned the empowerment of women and other disenfranchised groups, stemming, her associates said on Monday, from the quiet pride she took in her life as a gay woman.

Ms. Friedman is survived by her mother, Freda, and two sisters, Cheryl Friedman and Barbara Egli.

She was the subject of a documentary film, “A Journey of Spirit,” which followed her from 1997 to 2002.

If Ms. Friedman never attained the vast crossover success of Amy Grant, the Christian pop singer with whom she was often compared, it did not seem to bother her. In an interview with The Palm Beach Post in 2004, Ms. Friedman recounted her response to a music-industry executive who accused her of being just a big fish in a small pond.

“I’m not a fish,” Ms. Friedman replied.