Thorn or þorn (Þ, þ) is a letter in the Old English, Old Norse, Old Swedish and modern Icelandic alphabets, as well as modern transliterations of the Gothic alphabet, Middle Scots, and some dialects of Middle English. It was also used in medieval Scandinavia, but was later replaced with the digraph th, except in Iceland, where it survives. The letter originated from the rune ᚦ in the Elder Fuþark and was called thorn in the Anglo-Saxon and thorn or thurs in the Scandinavian rune poems. It is similar in appearance to the archaic Greek letter sho (ϸ), although the two are historically unrelated. The only language in which þ is currently in use is Icelandic.

It is pronounced as either a voiceless dental fricative [θ] or its voiced counterpart [ð]. However, in modern Icelandic, it is pronounced as a laminal voiceless alveolar non-sibilant fricative [θ̠], similar to th as in the English word thick, or a (usually apical) voiced alveolar non-sibilant fricative [ð̠], similar to th as in the English word the. Modern Icelandic usage generally excludes the latter, which is instead represented with the letter eth ⟨Ð, ð⟩; however, [ð̠] may occur as an allophone of /θ̠/, and written ⟨þ⟩, when it appears in an unstressed pronoun or adverb after a voiced sound.

In typography, the lowercase thorn character is unusual in that it has both an ascender and a descender (other examples are the lowercase Cyrillic ф, and, in some [especially italic] fonts, the Latin letters f and ſ [long s]).