In an alphabetic writing system, a silent letter is a letter that, in a particular word, does not correspond to any sound in the word's pronunciation.

In English, examples of silent letters include the "p" in "psychology," the "e" in "hope," and the "n" in "damn." The "t"s in "often" and "tsunami" are silent in some speakers' pronunciation of the words but spoken in others'.

Letters that are part of clusters that represent a single phoneme, such as the "s" and "h" in "shop," are usually not considered to be silent.

Silent letters are often relics of archaic or foreign pronunciations. The silent "h" in English words such as "rhyme" and "rhetoric," for example, originates from the use in Latin of the "rh" cluster to represent the roughly aspirated ῥ (rho) in Ancient Greek.

Silent letters create problems for both native and non-native speakers of a language, as they make it more difficult to guess the spellings of spoken words or the pronunciations of written words. Newly developed alphabets for previously unwritten languages and for planned languages such as Esperanto are thus typically designed to have few or no silent letters.