威廉·亨利·斯利曼爵士 （Sir William Henry Sleeman，1788年8月8日 - 1856年2月10日）是一名英国士兵、英属印度行政管理官及圣殿骑士团的一员。
Sleeman was born in Stratton, Cornwall and joined theBengal Army in 1814. He began his administrative career in 1820 and rose quickly in the military and his magisterial duties throughout the following fifteen years. At some point, he became a member of the Templar Order and rose to the rank of Master Templar. He actively searched for ancient artifacts for his Order and found dinosaur fossils in the Narmada valley region in 1828. Although this discovery was of little interest to the Templars, it made Sleeman famous in the scientific community. He also met the explorer Alexander Burnes and recruited him into the Templar Order, gaining a valuable associate in his search for artifacts, including a Staff of Edenonce in the possession of Alexander the Great.
Sleeman became known for his suppression of the Thuggeeassassins in India between 1835 and 1839. While these actions were initially motivated by a belief that they were linked with the Assassin Brotherhood, he was never able to confirm this.
In June 1839, the Templar Francis Cotton was killed during a confontation after having fatally poisoned Maharaja Ranjit Singh of the Sikh Empire in Amritsar. Sleeman was appointed the Templar's successor and discovered notes about Cotton's activities, including a full dossier on the Assassin Arbaaz Mirand the presence of a First Civilization Temple underneath the Maharaja's summer palace.
Inside a safe, Sleeman found a Precursor box and a set of documents related to its supposed powers, although the death of Cotton meant that he could do little to act on this information. Notes suggested that a power source was needed for the box and Sleeman believed that Mir had stolen the Koh-i-Noor, leading him to focus his efforts on the Assassin Brotherhood located by Cotton in Amritsar.