Smith raised a stink about the incident on his Twitter page Sunday, saying "I'm way fat. ... If you look like me, you may be ejected from Southwest Air."
Southwest says customers must be able to fit safely and comfortably in one seat or make other arrangements.
After a storm of angry online comments, the airline issued a "heartfelt apology" to Smith in a statement on its website.
Smith is the director of the films "Clerks" and "Chasing Amy." and often appears in his movies as a character named Silent Bob.
