Katakana (片仮名、カタカナ; IPA: [katakaꜜna, kataꜜkana]) is a Japanese syllabary, one component of the Japanese writing system along with hiragana, kanji and in some cases the Latin script (known as rōmaji).

The word katakana means "fragmentary kana", as the katakana characters are derived from components or fragments of more complex kanji. Katakana and hiragana are both kana systems. With one or two minor exceptions, each syllable (strictly mora) in the Japanese language is represented by one character or kana in each system. Each kana represents either a vowel such as "a" (katakana ア); a consonant followed by a vowel such as "ka" (katakana カ); or "n" (katakana ン), a nasal sonorant which, depending on the context, sounds like English m, n or ng ([ŋ]) or like the nasal vowels of Portuguese or Galician.

In contrast to the hiragana syllabary, which is used for Japanese words not covered by kanji and for grammatical inflections, the katakana syllabary usage is comparable to italics in English; specifically, it is used for transcription of foreign-language words into Japanese and the writing of loan words (collectively gairaigo); for emphasis; to represent onomatopoeia; for technical and scientific terms; and for names of plants, animals, minerals and often Japanese companies.

Katakana evolved from Japanese Buddhist monks transliterating Chinese texts into Japanese.

The complete katakana script consists of 48 characters, not counting functional and diacritic marks:

These are conceived as a 5×10 grid (gojūon, 五十音, literally "fifty sounds"), as shown in the adjacent table, read ア (a), イ (i), ウ (u), エ (e), オ (o), カ (ka), キ (ki), ク (ku), ケ (ke), コ (ko) and so on. The gojūon inherits its vowel and consonant order from Sanskrit practice. In vertical text contexts, which used to be the default case, the grid is usually presented as 10 columns by 5 rows, with vowels on the right hand side and ア (a) on top. Katakana glyphs in the same row or column do not share common graphic characteristics. Three of the syllabograms to be expected, yi, ye and wu, may have been used idiosyncratically with varying glyphs, but never became conventional in any language and are not present at all in modern Japanese.

The 50-sound table is often amended with an extra character, the nasal ン (n). This can appear in several positions, most often next to the N signs or, because it developed from one of many mu hentaigana, below the u column. It may also be appended to the vowel row or the a column. Here, it is shown in a table of its own.

The script includes two diacritic marks placed at the upper right of the base character that change the initial sound of a syllabogram. A double dot, called dakuten, indicates a primary alteration; most often it voices the consonant: k→g, s→z, t→d and h→b; for example, カ (ka) becomes ガ (ga). Secondary alteration, where possible, is shown by a circular handakuten: h→p; For example; ハ (ha) becomes パ (pa). Diacritics, though used for over a thousand years, only became mandatory in the Japanese writing system in the second half of the 20th century. Their application is strictly limited in proper writing systems,^{[clarification needed]} but may be more extensive in academic transcriptions.