Ian Robertson Underwood (born May 22, 1939) is a woodwind and keyboards player, who was a member of the original version of Frank Zappa's band the Mothers of Invention. Following the original band's split in late 1969, Underwood continued to work with Zappa extensively during the 1970s.

Underwood graduated from The Choate School in 1957 and Yale University with a bachelor's degree in composition in 1961 and a master's degree in composition at UC Berkeley in 1966. He began his career by playing San Francisco Bay Area coffeehouses and bars with his improvisational group, the Jazz Mice, in the mid-1960s before he became a member of Frank Zappa and the Mothers of Invention in 1967 for their third studio album, We're Only in It for the Money. He speaks on Uncle Meat; on the track "Ian Underwood Whips It Out" he relates how he first met Zappa and demonstrated his capabilities on the saxophone at Zappa's invitation. Underwood later worked with Frank Zappa on his solo recordings, including 1969's Hot Rats. He married Ruth Komanoff (Underwood), marimbist/percussionist from the Mothers of Invention in May 1969. Underwood left the Mothers of Invention in September 1973. He and Ruth divorced in 1986.

After his association with Frank Zappa, he pursued a career as a session keyboardist. Underwood has since been proficient on the Minimoog synthesizer, mostly in film. He has been credited in recordings for Quincy Jones, Barbra Streisand, Ronee Blakley, Hugh Cornwell, Freddie Hubbard, Jean-Luc Ponty, Herb Alpert, Hugh Masekela, Peggy Lee, Dolly Parton, Chicago, Janet Jackson, Dave Grusin, Jefferson Airplane, Frankie Valli, the Carpenters, James Ingram, and Barry Manilow. Underwood was also one of the musicians who played the main title theme for the 1980s hit series Knight Rider. Underwood was the uncredited producer of the debut album by Alice Cooper, Pretties For You, in 1969.

Underwood contributed synthesizers and programming to the historic recording of the Michael Jackson/Lionel Richie single "We Are the World" (produced by Quincy Jones in 1985); he has also been a featured performer (mostly on keyboard) with James Horner on numerous James Horner film scores including Titanic (1997) and Sneakers (1992).