Chi (/kaɪ/ ^ⓘ KY, also /xiː/ KHEE; uppercase Χ, lowercase χ; Greek: χῖ) is the twenty-second letter of the Greek alphabet.

Its value in Ancient Greek was an aspirated velar stop /kʰ/ (in the Western Greek alphabet: /ks/).

In Koine Greek and later dialects it became a fricative ([x]/[ç]) along with Θ and Φ.

In Modern Greek, it has two distinct pronunciations: In front of high or front vowels (/e/ or /i/) it is pronounced as a voiceless palatal fricative [ç], as in German ich or like some pronunciations of "h" in English words like hew and human. In front of low or back vowels (/a/, /o/ or /u/) and consonants, it is pronounced as a voiceless velar fricative ([x]), as in German ach or Spanish j. This distinction corresponds to the ich-Laut and ach-Laut of German.

Chi is romanized as ⟨ch⟩ in most systematic transliteration conventions, but sometimes ⟨kh⟩ is used. In addition, in Modern Greek, it is often also romanized as ⟨h⟩ or ⟨x⟩ in informal practice.

In the system of Greek numerals, it has a value of 600.

In ancient times, some local forms of the Greek alphabet used the chi instead of xi to represent the /ks/ sound. This was borrowed into the early Latin language, which led to the use of the letter X for the same sound in Latin, and many modern languages that use the Latin alphabet.

Chi was also included in the Cyrillic script as the letter Х, with the phonetic value /x/ or /h/.