The king of Babylon (Akkadian: šakkanakki Bābili, later also šar Bābili) was the ruler of the ancient Mesopotamian city of Babylon and its kingdom, Babylonia, which existed as an independent realm from the 19th century BC to its fall in the 6th century BC. For the majority of its existence as an independent kingdom, Babylon ruled most of southern Mesopotamia, composed of the ancient regions of Sumer and Akkad. The city experienced two major periods of ascendancy, when Babylonian kings rose to dominate large parts of the Ancient Near East: the First Babylonian Empire (or Old Babylonian Empire, c. 1894/1880–1595 BC) and the Second Babylonian Empire (or Neo-Babylonian Empire, 626–539 BC). Babylon was ruled by Hammurabi, who created the Code of Hammurabi.

Many of Babylon's kings were of foreign origin. Throughout the city's nearly two-thousand year history, it was ruled by kings of native Babylonian (Akkadian), Amorite, Kassite, Elamite, Aramean, Assyrian, Chaldean, Persian, Greek and Parthian origin. A king's cultural and ethnic background does not appear to have been important for the Babylonian perception of kingship, the important matter instead being whether the king was capable of executing the duties traditionally ascribed to the Babylonian king: establishing peace and security, upholding justice, honouring civil rights, refraining from unlawful taxation, respecting religious traditions, constructing temples, providing gifts to the gods in the temples and maintaining cultic order. Babylonian revolts of independence during the times the city was ruled by foreign empires probably had little to do with the rulers of these empires not being Babylonians and more to do with the rulers rarely visiting Babylon and failing to partake in the city's rituals and traditions.

Babylon's last native king was Nabonidus, who reigned from 556 to 539 BC. Nabonidus's rule was ended through Babylon being conquered by Cyrus the Great of the Achaemenid Empire. Though early Achaemenid kings continued to place importance on Babylon and continued using the title 'king of Babylon', later Achaemenid rulers being ascribed the title is probably only something done by the Babylonians themselves, with the kings themselves having abandoned it. Babylonian scribes continued to recognise rulers of the empires that controlled Babylonia as their kings until the time of the Parthian Empire, when Babylon was gradually abandoned. Though Babylon never regained independence after the Achaemenid conquest, there were several attempts by the Babylonians to drive out their foreign rulers and re-establish their kingdom, possibly as late as 336/335 BC under the rebel Nidin-Bel.

Throughout the city's long history, various titles were used to designate the ruler of Babylon and its kingdom, the most common titles being 'viceroy of Babylon', 'king of Karduniash' and 'king of Sumer and Akkad'. Use of one of the titles did not mean that the others could not be used simultaneously. For instance, the Neo-Assyrian king Tiglath-Pileser III (r. 729–727 BC in Babylon), used all three of the aforementioned titles.

The Babylonian kings derived their right to rule from divine appointment by Babylon's patron deity Marduk and through consecration by the city's priests. Marduk's main cult image (often conflated with the god himself), the statue of Marduk, was prominently used in the coronation rituals for the kings, who received their crowns "out of the hands" of Marduk during the New Year's festival, symbolizing them being bestowed with kingship by the deity. The king's rule and his role as Marduk's vassal on Earth were reaffirmed annually at this time of year, when the king entered the Esagila, Babylon's main cult temple, alone on the fifth day of the New Year's Festival each year and met with the high priest. The high priest removed the regalia from the king, slapped him across the face and made him kneel before Marduk's statue. The king would then tell the statue that he had not oppressed his people and that he had maintained order throughout the year, whereafter the high priest would reply (on behalf of Marduk) that the king could continue to enjoy divine support for his rule, returning the royal regalia. Through being a patron of Babylon's temples, the king extended his generosity towards the Mesopotamian gods, who in turn empowered his rule and lent him their authority.

Babylonian kings were expected to establish peace and security, uphold justice, honor civil rights, refrain from unlawful taxation, respect religious traditions and maintain cultic order. None of the king's responsibilities and duties required him to be ethnically or even culturally Babylonian. Any foreigner sufficiently familiar with the royal customs of Babylonia could adopt the title, though they might then require the assistance of the native priesthood and the native scribes. Ethnicity and culture does not appear to have been important in the Babylonian perception of kingship: many foreign kings enjoyed support from the Babylonians and several native kings were despised. That the rule of some foreign kings was not supported by the Babylonians probably has little to do with their ethnic or cultural background, but rather that they were perceived as not properly executing the traditional duties of the Babylonian king.

As with other monarchies, the kings of Babylon are grouped into a series of royal dynasties, a practice started by the ancient Babylonians themselves in their king lists. The generally accepted Babylonian dynasties should not be understood as familial groupings in the same vein as the term is commonly used by historians for ruling families in later kingdoms and empires. Though Babylon's first dynasty did form a dynastic grouping where all monarchs were related, the dynasties of the first millennium BC, notably the Dynasty of E, did not constitute a series of coherent familial relationships at all. In a Babylonian sense, the term dynasty, rendered as palû or palê, related to a sequence of monarchs from the same ethnic or tribal group (i.e. the Kassite dynasty), the same region (i.e. the dynasties of the Sealand) or the same city (i.e. the dynasties of Babylon and Isin). In some cases, kings known to be genealogically related, such as Eriba-Marduk (r. c. 769–760 BC) and his grandson Marduk-apla-iddina II (r. 722–710 BC and 703 BC), were separated into different dynasties, the former designated as belonging to the Dynasty of E and the latter as belonging to the (Third) Sealand dynasty.

Among all the different types of documents uncovered through excavations in Mesopotamia, the most important for reconstructions of chronology and political history are king-lists and chronicles, grouped together under the term 'chronographic texts'. Mesopotamian king lists are of special importance when reconstructing the sequences of monarchs, as they are collections of royal names and regnal dates, also often with additional information such as the relations between the kings, arranged in a table format. In terms of Babylonian rulers, the main document is the Babylonian King List (BKL), a group of three independent documents: Babylonian King List A, B, and C. In addition to the main Babylonian King Lists, there are also additional king-lists that record rulers of Babylon.