The 23-year-old singer was scheduled to sing at the Los Angeles County Museum of Art's (LACMA) Art + Film Gala on November 7.
However, he pulled out of the event after being ordered by doctors to go on vocal rest, said the Hollywood Reporter.
A representative for the museum, Miranda Carroll, said, "Sam Smith has fallen ill and regretfully was unable to perform tonight at the LACMA Art + Film Gala. He was required to go on vocal rest."
Singer Joe Walsh stepped in at the last minute to replace Smith.
Celebrities who were spotted at the event included LACMA co-chair Leonardo Dicaprio, Gwyneth Paltrow, Reese Witherspoon, Jared Leto, and Aaron and Sam Taylor Johnson.
