(reigned c. 1650–c. 1620 BC), early king of the Hittite Old Kingdom in Anatolia.

The son of the preceding king, Labarnas I, Hattusilis was also at first called Labarnas but apparently assumed his new name after he transferred his capital from Kussara to Hattusa (Boğazköy).

Unlike Labarnas I, who concentrated on uniting the Hittite heartland, Hattusilis penetrated the Taurus range and invaded northern Syria, attacking the kingdom of Yamhad. His reign was troubled by domestic intrigue, leading him to disown three rebellious sons in the line of succession. In his “Farewell Address,” intended for his kin and the pankus (a general assembly), Hattusilis reviewed past political conflicts, designated his grandson Mursilis I as his successor, and exhorted his family and people to observe virtue and moderation. This address is an important source for the political history of the Hittite Old Kingdom.