"Ä" and "ä" are both characters that represent either a letter from several extended Latin alphabets, or the letter A with an umlaut mark or diaeresis.

The letter Ä occurs as an independent letter in the Swedish, Finnish, Skolt Sami, Karelian, Estonian, Luxembourgish, North Frisian, Saterlandic, Emiliano-Romagnolo, Rotuman, French, Slovak, Tatar, Gagauz, and Turkmen alphabets, where it represents a vowel sound. In Finnish and Turkmen this is always /æ/; in Swedish and Estonian, regional variation, as well as the letter's position in a word, allows for either [æ] and [ɛ]. In German and Slovak Ä stands for [ɛ] (or a bit archaic but still correct [æ]).

In the Nordic countries, the vowel sound [æ] was originally written as "Æ" when Christianisation caused the former Vikings to replace the Runic alphabet with the Latin alphabet around A.D. 1100. The letter Ä arose in German and later in Swedish from originally writing the E in AE on top of the A, which with time became simplified as two dots. In the Icelandic, Faroese, Danish and Norwegian alphabets, "Æ" is still used instead of Ä.

Finnish later adopted the Swedish alphabet during the 700 years that Finland was part of Sweden. Although the phenomenon of Germanic umlaut does not exist in Finnish, the phoneme /æ/ does. Estonian gained the letter through high and extensive exposure to Germanic languages with Swedish in-particular during the 160 years of Estonia as an occupied, later integrated part of the Swedish Empire.

The letter is used in some Romani alphabets as well.