Ismah Khatun (Persian: اسماعه خاتون) (Arabic: عصمة خاتون) was a Seljuk princess, daughter of sultan Malik Shah (r. 1072–1092) and principal wife of Abbasid caliph al-Mustazhir (r. 1094–1118).
Ismah Khatun was one of the youngest daughter of Seljuk sultan Malik Shah.
She was very young when her father died in 1092, he was succeeded by his underage son Mahmud I under the regency of Terken Khatun, who was the regent during his minority in 1092–1094.
On 3 February 1112, she gave birth to prince Abu Ishaq Ibrahim, who died of smallpox in October 1114, and was buried in the mausoleum of al-Muqtadir in Rusafah Cemetery, beside his uncle-cousin Ja'far, son of the caliph al-Muqtadi (father of Mustazhir) and Mah-i Mulk Khatun (half-sister of Ismah).
Upon the death of Al-Mustazhir, Ismah returned to Isfahan, where she died, and was buried within the law college that she had founded there on Barracks Market Street.