Only in the US. From Wikipedia:
The common misconception that people before the age of exploration believed that Earth was flat entered the popular imagination after Washington Irving's publication of The Life and Voyages of Christopher Columbus in 1828. In the United States, this belief persists in the popular imagination, and is even repeated in some widely read textbooks. Previous editions of Thomas Bailey's The American Pageant stated that "The superstitious sailors ... grew increasingly mutinous...because they were fearful of sailing over the edge of the world"; however, no such historical account is known. Actually, sailors were probably among the first to know of the curvature of Earth from daily observations — seeing how shore landscape features (or masts of other ships) gradually descend/ascend near the horizon.