Quarto (abbreviated 4to or 4°) is a book or pamphlet produced from full 'blanksheets', each of which is printed with eight pages of text, four to a side, then folded two times to produce four leaves (that is, eight book pages). Each printed page now presents as one-fourth size of the full blanksheet.

The earliest known European printed book is a quarto, the Sibyllenbuch, believed to have been printed by Gutenberg in 1452–53, before the Gutenberg Bible, surviving only as a fragment. Quarto is also used as a general description of size of books that are about 12 inches tall, and as such does not necessarily indicate the actual printing format of the books, which may even be unknown as is the case for many modern books. These terms are discussed in greater detail in Book sizes.

A quarto (from Latin quartō, ablative form of quartus, fourth ) is a book or pamphlet made up of one or more full sheets of paper on which 8 pages of text were printed, which were then folded two times to produce four leaves. Each leaf of a quarto book thus represents one fourth the size of the original sheet. Each group of 4 leaves (called a "gathering" or "quire") could be sewn through the central fold to attach it to the other gatherings to form a book. Sometimes, additional leaves would be inserted within another group to form, for example gatherings of 8 leaves, which similarly would be sewn through the central fold. Generally, quartos have more squarish proportions than folios or octavos.