Shams al-Din Ildeniz, Eldigüz or Shamseddin Eldeniz (Azerbaijani: شمس الدین الدنگز, شمس الدین الدنیز Şəmsəddin Eldəniz, died c. 1175-1176) was an atabeg of the Seljuq empire and founder of the dynasty of Eldiguzids (Atabegs of Azerbaijan), which held sway over Caucasian Albania, Iranian Azerbaijan, and most of northwestern Persia from the second half of the 12th century to the early decades of the 13th.
A Kipchak by origin, he was formerly a freedman of Seljuq sultan Mehmed I's (1118-1131) vizier Kamal Din al-Simirumi and attained to the post of governor of Arran under Sultan Mas'ud (1134-1152). His raise as the most powerful peripheral emirs of the Seljuq empire was aided by the necessity of having a large army against the frequent incursions from the neighboring kingdom of Georgia. He made himself virtually independent ruler of Azerbaijan by 1146. His marriage with the widow of Sultan Toghrul II (1132-1133; Masud’s brother and predecessor) afforded him to intervene in the dynastic strife which erupted upon Mas'ud’s death in 1152. He succeeded, in 1160, in deposing Sulayman Shah and installing his stepson Arslan b. Toghrul (1160-1175) as sultan. Conferred with the rank of atabek, Ildeniz now became a chief protector of the sultan’s authority.
Ildeniz then arranged a marriage between his son Pahlawan and the daughter of Inanch, amir of Rey, in order to secure the allegiance of this powerful dynast. Later Inanch allied himself with the amirs of Fars and Qazvin and attempted to depose Arslan in favor of his brother Muhammad. Ildeniz met the renegades on a battlefield and won a victory, but Inanch escaped to Rey. Ildeniz then marched to Isfahan and forced the atabek of Fars, Zangi, into submission. Soon he proceeded northward to recover the city of Dvin from the Georgian attack in 1162. A coalition of Muslim rulers led by Ildeniz defeated the Georgian king Giorgi III and forced him to withdraw into his possessions. Back at Hamadan, he had to deal with another invasion – this time by the Khwarezmians who planned to annex Khorasan. The Khwarezminas avoided the confrontation and retreated in the face of the advancing army of Ildeniz. Their ally Inanch was murdered at the request of Ildeniz in 1169. It was not, however, until the death of the Khwarazmshah Il-Arslan in 1172, when the threats on this sector were finally eliminated.
By the time of his death around 1175-1176, Ildeniz was arguably the undisputed de facto master of many parts of the already fragmentized Great Seljuq Empire, centered on Iraq. He was buried at Hamadan, at a madrasa which he had founded.