The Kronos Quartet is an American string quartet based in San Francisco. It has been in existence with a rotating membership of musicians for 50 years. The quartet covers a very broad range of musical genres, including contemporary classical music. More than 1,200 works have been written for it. The quartet has recorded over 75 albums and received a number of awards.

The quartet was founded by violinist David Harrington in Seattle, Washington. Its first performance was in November 1973. Since 1978, the quartet has been based in San Francisco, California. It is named after the Greek Titan Kronos. The longest-running combination of performers (from 1978 to 1999) had Harrington and John Sherba on violin, Hank Dutt on viola, and Joan Jeanrenaud on cello. In 1999, Jeanrenaud left Kronos because she was "eager for something new"; she was replaced by Jennifer Culp, who, in turn, left in 2005 and was replaced by Jeffrey Zeigler. In June 2013, Zeigler was replaced by Sunny Yang. In February 2023, cellist and composer Paul Wiancko became the quartet's newest cellist. In March 2024, Kronos Quartet announced that Sherba and Dutt would retire in June of that year. They were replaced by violinist Gabriela Díaz and violist Ayane Kosaza.

With over 75 studio albums to its credit and having performed worldwide, the Kronos Quartet has been called "probably the most famous 'new music' group in the world" and been praised in philosophical studies of music for the inclusiveness of its repertoire.

By the time the quartet celebrated its 25th anniversary in 1999, it had a repertoire of over 600 works, including 400 quartets written for it, more than 3,000 performances, seven first-prize ASCAP awards, Edison Awards in classical and popular music, and had more than 1.5 million record sales.

When Kronos turned 30, in 2003, it decided to commission new pieces from composers under age 30, in hopes of encouraging talented young composers. The program, called the Under 30 Project, is now run in cooperation with Carnegie Hall, Cal Performances at the University of California, Berkeley, and the Montalvo Arts Center. The first recipient was Alexandra du Bois (at the time a student at Indiana University, later a Juilliard School graduate), followed by Felipe Perez Santiago (born in Mexico in 1973), and Dan Visconti (born in Illinois in 1982); in 2007, Israeli composer Aviya Kopelman became the fourth.

To celebrate its 40th year, the Kronos Quartet returned to Seattle, the city in which it first played, and worked in collaboration with Seattle's Degenerate Art Ensemble to create a piece incorporating music, dance and video. It celebrated its 40th anniversary with a sold-out performance at Zellerbach Hall, UC Berkeley, in December 2013. The same year, Michael Giacchino, a soundtrack composer who often names his pieces with puns, published a piece named after Kronos, "The Kronos Wartet", as a part of the soundtrack to Star Trek Into Darkness for a scene that takes place on the fictional planet "Kronos". (also spelled "Qo'noS").

Over 1,000 pieces have been created for the Kronos Quartet, which has a long history of commissioning new works. It has worked with many minimalist composers, including John Adams, Arvo Pärt, George Crumb, Henryk Górecki, Steve Reich, John Luther Adams, Roberto Paci Dalò, Philip Glass, Terry Riley, Peter Sculthorpe and Kevin Volans; collaborators hail from a diversity of countries—Kaija Saariaho from Finland, Pēteris Vasks from Latvia, Franghiz Ali-Zadeh from Azerbaijan, Homayun Sakhi from Afghanistan, Hamza El Din from Egypt, Victoria Vita Polevá from Ukraine and Fernando Otero, Astor Piazzolla, and Osvaldo Golijov from Argentina. Some of Kronos's string-quartet arrangements were published in 2007.

I've always wanted the string quartet to be vital, and energetic, and alive, and cool, and not afraid to kick ass and be absolutely beautiful and ugly if it has to be. But it has to be expressive of life. To tell the story with grace and humor and depth. And to tell the whole story, if possible.