Eth (/ɛð/ edh, uppercase: ⟨Ð⟩, lowercase: ⟨ð⟩; also spelled edh or eð), known as ðæt (that) in Old English, is a letter used in Old English, Middle English, Icelandic, Faroese (in which it is called edd), and Elfdalian alphabets.

It was also used in Scandinavia during the Middle Ages, but was subsequently replaced with ⟨dh⟩, and later ⟨d⟩.

It is often transliterated as ⟨d⟩.

The lowercase version has been adopted to represent a voiced dental fricative (IPA: [ð]) in the International Phonetic Alphabet.

In Faroese, ⟨ð⟩ is not assigned to any particular phoneme and appears mostly for etymological reasons, but it indicates most glides. When ⟨ð⟩ appears before ⟨r⟩, it is in a few words pronounced [ɡ]. In the Faroese alphabet, ⟨ð⟩ follows ⟨d⟩.

⟨Ð⟩ is sometimes used in Khmer romanization to represent ឍ thô.

In Icelandic, ⟨ð⟩, called "eð", represents an alveolar non-sibilant fricative, voiced [ð̠] intervocalically and word-finally, and voiceless [θ̠] otherwise, which form one phoneme, /θ/. Generally, /θ/ is represented by thorn ⟨Þ⟩ at the beginning of words and by ⟨ð⟩ elsewhere. The ⟨ð⟩ in the name of the letter is devoiced in the nominative and accusative cases: [ɛθ̠]. In the Icelandic alphabet, ⟨ð⟩ follows ⟨d⟩.

In Olav Jakobsen Høyem's version of Nynorsk based on Trøndersk, ⟨ð⟩ was always silent, and was introduced for etymological reasons.