Atsïz (Persian: علاء الدين أتسز; full name: Ala ad-Dunya wa ad-Din Abul-Muzaffar Atsiz Qizil Arslan ibn Muhammad) was the second Shah of Khwarezm from 1127 to 1156. He was the son of Qutb ad-Din Muhammad.
Atsïz gained his position following his father's death in 1127. During the early part of his reign he focused on securing Khwarazm against nomad attacks. In 1138 he rebelled against his suzerain, the Seljuk sultan Ahmed Sanjar, but was defeated in Hezarasp and forced to flee. Sanjar installed his nephew Suleiman Shah as ruler of Khwarazm and returned to Merv. Atsïz returned, however, and Suleiman Shah was unable to hold onto the province. Atsïz then attacked Bukhara, but by 1141 he again submitted to Sanjar, who pardoned him and formally returned control of Khwarazm over to him.
The same year that Sanjar pardoned Atsïz, the Kara Khitai under Yelü Dashi defeated the Seljuks at Qatwan, near Samarkand. Atsïz took advantage of the defeat to invade Khurasan, occupying Merv and Nishapur. Yelü Dashi, however, sent a force to plunder Khwarazm, forcing Atsïz to pay an annual tribute.
In 1142 Atsiz was expelled from Khurasan by Sanjar, who invaded Khwarazm in the following year and forced Atsïz back into vassalage, although Atsïz continued to pay tribute to the Kara Khitai until his death. Sanjar undertook another expedition against Atsïz in 1147 when the latter became rebellious again.
In 1153 Sanjar was defeated and imprisoned by a collection of Oghuz tribes, and Khurasan soon descended into anarchy. The portion of the Seljuk army that refused to join the Oghuz proclaimed the former ruler of the Karakhanids, Mahmud Khan, as their leader. Mahmud sought an alliance with Atsïz against the Oghuz, while Atsïz's brother Ïnal-Tegin had already in 1154 plundered a part of Khurasan. Atsïz and his son Il-Arslan departed from Khwarazm, but before they could make any gains Sanjar escaped from his captivity and restored his rule. Atsïz died soon afterwards, and was succeeded in Khwarazm by Il-Arslan.