Šamaš-šuma-ukin (Neo-Assyrian Akkadian: 𒌋𒌋𒈬𒁺, romanized: Šamaš-šuma-ukin or Šamaš-šumu-ukīn, meaning "Shamash has established the name"), was king of Babylon as a vassal of the Neo-Assyrian Empire from 668 BC to his death in 648. Born into the Assyrian royal family, Šamaš-šuma-ukin was the son of the Neo-Assyrian king Esarhaddon and the elder brother of Esarhaddon's successor Ashurbanipal.

Despite being the elder son, Šamaš-šuma-ukin was for unknown reasons bypassed as heir to Assyria. His designation as heir to Babylonia was likely devised by Esarhaddon as a means to counteract future rivalry and jealousy between the brothers. Although Esarhaddon specified that Šamaš-šuma-ukin was to swear an oath of allegiance to Ashurbanipal, the clear primary heir, Šamaš-šuma-ukin was also referred to as Ashurbanipal's "equal brother" and Ashurbanipal was to stay out of his affairs. This part of the succession plans were not upheld by Ashurbanipal after Esarhaddon's death; Šamaš-šuma-ukin only acceded to the Babylonian throne months after Ashurbanipal had become king and was throughout his reign a closely monitored vassal, not entrusted with all of Babylonia or substantial military forces and only allowed to make decisions if they were approved and verified by Ashurbanipal.

Šamaš-šuma-ukin assimilated well into Babylonia, despite being ethnically and culturally Assyrian. His royal inscriptions are far more "quintessentially Babylonian" than those of other Assyrian rulers of southern Mesopotamia, using Babylonian imagery and rhetoric to an unprecedented extent. He participated in the Babylonian New Year's festival and is recorded as partaking in other Babylonian traditions. The Statue of Marduk, the main cult image of Babylon's patron deity Marduk, was returned to Babylon in 668 at Šamaš-šuma-ukin's coronation, having been stolen from the city by his grandfather Sennacherib twenty years prior.

Though Šamaš-šuma-ukin maintained peaceful relations with his younger brother for many years, resentment gradually grew between them due to Ashurbanipal's overbearing control. In 652, Šamaš-šuma-ukin revolted, inspiring the Babylonians to join him and recruiting a coalition of enemies of Assyria, including the Elamites, Chaldeans, Arameans and perhaps the Medes. Though the conflict was initially indecisive, it eventually ended in disaster for Šamaš-šuma-ukin. Babylon was captured by Ashurbanipal in 648 after a lengthy siege and Šamaš-šuma-ukin died, though the exact circumstances of his death are unclear. After his defeat and death there is evidence of a large-scale damnatio memoriae campaign, with images of the king being mutilated, erasing his face.

Šamaš-šuma-ukin was probably the second eldest son of Esarhaddon, the third king of the Sargonid dynasty, younger only than the crown prince Sin-nadin-apli. Upon the unexpected death of Sin-nadin-apli in 674 BC, the Assyrian court was thrown into upheaval. Esarhaddon had only become king through great difficulty and wished to avoid his own eventual death initiating a succession crisis. He thus soon began to draw up new succession plans. Esarhaddon decided that the fourth-eldest son Ashurbanipal would inherit Assyria, clearly the primary title. Šamaš-šuma-ukin, the next-eldest son after Sin-nadin-apli, was instead designated as the heir to Babylonia, with the two designated as "equal brothers". The third eldest son Šamaš-metu-uballiṭ was entirely bypassed, perhaps because he suffered from poor health. Despite proclaiming his two sons as equals, Esarhaddon also made it clear that Šamaš-šuma-ukin was to swear an oath of allegiance to Ashurbanipal.

After Esarhaddon made his decision, the two princes arrived at the Assyrian capital of Nineveh together and partook in a celebration in May 672 with foreign representatives, Assyrian nobles and elements of the military. Promoting one son as heir to Assyria and the other as heir to Babylonia was a novel idea; for the past decades the Assyrian king had simultaneously ruled Babylonia. The decision to bypass Šamaš-šuma-ukin as heir to Assyria was also a remarkable one, given that Esarhaddon's own accession issues had been the direct result of his father Sennacherib acting in a similar way. Sennacherib had bypassed the elder son Arda-Mulissu in favor of the younger Esarhaddon; in Sennacherib's case this decision led to Arda-Mulissu murdering him and fighting a civil war against Esarhaddon. Why Esarhaddon made nearly the same decision is not clear.

One hypothesis in regard to why a younger son was designated as the heir to what was clearly Esarhaddon's primary title is that Ashurbanipal and Šamaš-šuma-ukin could have had different mothers. Though it is equally likely that Šamaš-šuma-ukin and Ashurbanipal shared a mother, possibly Ešarra-ḫammat (Esarhaddon's primary consort), it is also possible that Ashurbanipal was the son of an Assyrian woman and Šamaš-šuma-ukin was the son of a Babylonian woman; Šamaš-šuma-ukin acceding to the Assyrian throne could thus have been problematic. The decision to grant Šamaš-šuma-ukin Babylonia and designate him and Ashurbanipal as "equal brothers" could perhaps be due to Esarhaddon wishing to avoid the rivalry and jealousy involved in his own accession. If Šamaš-šuma-ukin had Babylonian heritage, Esarhaddon could have surmised that he was a superior candidate for the Babylonian throne.

As crown prince, Šamaš-šuma-ukin would have undergone training for traditional royal duties, such as hunting, riding, scholarship and wisdom, archery, chariotry and other forms of military training. Because Esarhaddon was constantly ill, much of the administrative duties of the empire fell upon Ashurbanipal and Šamaš-šuma-ukin during the last few years of their father's reign. It was important for the crown princes to gain real experience in ruling. Letters of correspondence between Esarhaddon and Ashurbanipal from this time for instance show that Ashurbanipal prominently participated in the Assyrian intelligence network, gathering information on foreign enemies and rivals and compiling reports for his father.