Kashtiliash III (a.k.a. Kaštiliašu III) is a possible Kassite king of Babylonia in the 15th century B.C.E. (Short Chronology).
Evidence of Kaštiliašu's kingship is somewhat circumstantial. He may be the person indicated on line 12 of the Synchronistic King List who is preceded by a lacuna and superseded by a poorly preserved name which is unlikely to be Ulam-Buriaš. Two passages in the Chronicle of Early Kings mention Kaštiliašu: "Ulam-Buriaš, brother of Kaštiliašu, the Kassite" and "Agum, the son of Kaštiliašu". (Ulam-Buriaš conquered and ruled the Sealand—at the southern end of Babylonia—and perhaps ruled as king of Babylonia; Agum (III) was king of Babylonia.) He has no royal title in those, a feature of this chronicle that is shared by others, such as Samsu-Ditana, who did prove to be kings.