Ala al-Din wa-l-Dawla Abu'l-Muzaffar Atsiz ibn Muhammad ibn Anushtegin, better known as Atsiz (اتسز) was the second Khwarazmshah from 1127 to 1156. He was the son and successor of Muhammad I.

The Khwarazmian or Khwarezmian Empire was a culturally Persianate, Sunni Muslim empire of Turkic mamluk origin. Khwarazmians ruled large parts of present-day Central Asia, Afghanistan, and Iran from 1077 to 1231; first as vassals of the Seljuk Empire and the Qara Khitai, and from circa 1190 as independent rulers up until the Mongol conquest in 1219–1221.