Jonathan Richard Guy Greenwood (born 5 November 1971) is an English musician who is the lead guitarist and keyboardist of the rock band Radiohead. He has also composed numerous film scores. He has been named one of the greatest guitarists by numerous publications, including Rolling Stone.

Along with his elder brother, Colin, Greenwood attended Abingdon School in Abingdon near Oxford, where he formed Radiohead. Their debut single, "Creep" (1992), was distinguished by Greenwood's aggressive guitar work. Radiohead have achieved acclaim and sold more than 30 million albums. He was inducted into the Rock and Roll Hall of Fame as a member of Radiohead in 2019.

Greenwood is a multi-instrumentalist and a prominent player of the ondes Martenot, an early electronic instrument. He uses electronic techniques such as programming, sampling and looping, and writes music software. He described his role in Radiohead as an arranger, helping transform Thom Yorke's demos into finished songs. The only classically trained member of Radiohead, Greenwood has composed for orchestras including the London Contemporary Orchestra and the BBC Concert Orchestra, and his arrangements feature on Radiohead records.

Greenwood released his first solo work, the soundtrack for the film Bodysong, in 2003. In 2007, he scored There Will Be Blood, the first of several collaborations with the director Paul Thomas Anderson. In 2018, he was nominated for an Academy Award for his score for Anderson's Phantom Thread. He was nominated again for his score for The Power of the Dog (2021), directed by Jane Campion. Greenwood also scored the Lynne Ramsay films We Need to Talk About Kevin (2011) and You Were Never Really Here (2017). He has collaborated with Middle Eastern musicians including the Israeli songwriters Shye Ben Tzur and Dudu Tassa. In 2021, Greenwood debuted a new band, the Smile, with Yorke and the drummer Tom Skinner.

Jonny Greenwood was born on 5 November 1971 in Oxford, England. His brother, the Radiohead bassist Colin Greenwood, is two years older. Their father served in the British Army as a bomb disposal expert. The Greenwood family has historical ties to the Communist Party of Great Britain and the socialist Fabian Society.

When he was a child, Greenwood's family would listen to a small number of cassettes in their car, including Mozart's horn concertos, the musicals Flower Drum Song and My Fair Lady, and cover versions of Simon & Garfunkel songs. When the cassettes were not playing, Greenwood would listen to the noise of the engine and try to recall every detail of the music. He credited his older siblings with exposing him to rock bands such as the Beat and New Order. The first gig Greenwood attended was the Fall on their 1988 Frenz Experiment tour, which he found "overwhelming".

The Greenwood brothers attended the private boys' school Abingdon. The Abingdon director of music, Michael Stinton, recalled Jonny as a "charming student" and "committed musician" who would spend as much time in the music department as possible. Greenwood's first instrument was a recorder given to him at age four or five. He played baroque music in recorder groups as a teenager, and continued to play into adulthood. He played the viola in the Thames Vale youth orchestra, which he described as a formative experience: "I'd been in school orchestras and never seen the point. But in Thames Vale I was suddenly with all these 18-year-olds who could actually play in tune. I remember thinking: 'Ah, that's what an orchestra is supposed to sound like!'" Greenwood also spent time programming, experimenting with BASIC and simple machine code to make computer games. According to Greenwood, "The closer I got to the bare bones of the computer, the more exciting I found it."

At Abingdon, the Greenwood brothers formed a band, On a Friday, with the singer Thom Yorke, the guitarist Ed O'Brien and the drummer Philip Selway. Jonny, the youngest, was three school years below Yorke and Colin and the last to join. He was previously in another band, Illiterate Hands, with Matt Hawksworth, Simon Newton, Ben Kendrick, Nigel Powell and Yorke's brother, Andy.