Dayworld is a science fiction novel by American writer Philip José Farmer. Published in 1985, it is the first in the Dayworld tetralogy of novels inspired by Farmer's own 1971 short story "The Sliced-Crosswise Only-On-Tuesday World". There are two sequels - Dayworld Rebel (1987) and Dayworld Breakup (1990) - and one prequel, , co-authored by Danny Adams (2016).