U (う in hiragana or ウ in katakana) is one of the Japanese kana, each of which represents one mora. In the modern Japanese system of alphabetical order, they occupy the third place in the modern Gojūon (五十音) system of collating kana. In the Iroha, they occupied the 24th position, between む and ゐ. In the Gojūon chart (ordered by columns, from right to left), う lies in the first column (あ行, "column A") and the third row (う段, "row U"). Both represent the sound [ɯ]. In the Ainu language, the small katakana ゥ represents a diphthong, and is written as w in the Latin alphabet.

The hiragana form with dakuten, ゔ, representing the sound "v", is rarely seen on older words, since the sound does not occur in native Japanese words. However, it is becoming more common with Western influences.