The Island of the Day Before is a 1994 historical fiction novel by Umberto Eco set in the 17th century during the historical search for the secret of longitude. The central character is Roberto della Griva, an Italian nobleman stranded on a deserted ship in the Pacific Ocean, and his slowly decaying mental state, in a backdrop of Baroque-era science, metaphysics, and cosmology.

Umberto Eco was an Italian medievalist, philosopher, semiotician, novelist, cultural critic, and political and social commentator. In English, he is best known for his popular 1980 novel The Name of the Rose, a historical mystery combining semiotics in fiction with biblical analysis, medieval studies and literary theory, as well as Foucault's Pendulum, his 1988 novel which touches on similar themes.