Pythagoras, who lived in the 6th century B.C., concluded that the earth is round.
It is possible that some or many illiterate medieval peasants believed in a flat earth, but "[a]ccording to Stephen Jay Gould, 'there never was a period of "flat earth darkness" among scholars (regardless of how the public at large may have conceptualized our planet both then and now). Greek knowledge of sphericity never faded, and all major medieval scholars accepted the earth's roundness as an established fact of cosmology. 5. Gould, S.J. (1996). 'The late birth of a flat earth'. Dinosaur in a Haystack: Reflections in Natural History. New York: Crown: 38–52."
"The misconception that educated people at the time of Columbus believed in a flat Earth...was listed [in 1945] by the Historical Association (of Britain) as the second of 20 in a pamphlet on common errors in history." Flat Earth, Wikipedia (accessed October 11, 2010).