Ku (hiragana: く, katakana: ク) is one of the Japanese kana, which each represent one mora. Both represent [kɯ] and their shapes come from the kanji 久.

This kana may have a dakuten added, transforming it into ぐ in hiragana, グ in katakana and gu in Hepburn romanization. The dakuten's addition also changes the sound of the mora represented, to [ɡɯ] in initial positions and varying between [ŋɯ] and [ɣɯ] in the middle of words.

A handakuten (゜) does not occur with ku in normal Japanese text, but it may be used by linguists to indicate a nasal pronunciation [ŋɯ].

In the Ainu language, the katakana ク can be written as small ㇰ, representing a final k sound as in アイヌイタㇰ Ainu itak (Ainu language). This was developed along with other extended katakana to represent sounds in Ainu that are not found in standard Japanese katakana.