Alexander I Theopator Euergetes, surnamed Balas (Ancient Greek: Ἀλέξανδρος Βάλας, romanized: Alexandros Balas), was the ruler of the Seleucid Empire from 150 BC to August 145 BC. Picked from obscurity and supported by the neighboring Roman-allied Kingdom of Pergamon, Alexander landed in Phoenicia in 152 BC and started a civil war against Seleucid King Demetrius I Soter. Backed by mercenaries and factions of the Seleucid Empire unhappy with the existing government, he defeated Demetrius and took the crown in 150 BC. He married the princess Cleopatra Thea to seal an alliance with the neighboring Ptolemaic kingdom. His reign saw the steady retreat of the Seleucid Empire's eastern border, with important eastern satrapies such as Media being lost to the nascent Parthian Empire. In 147 BC, Demetrius II Nicator, the young son of Demetrius I, began a campaign to overthrow Balas, and civil war resumed. Alexander's ally, Ptolemaic king Ptolemy VI Philometor, moved troops into Coele-Syria to support Alexander, but then switched sides and threw his support behind Demetrius II. At the Battle of the Oenoparus River in Syria, he was defeated by Ptolemy VI and he died shortly afterward.

Alexander Balas claimed to be the son of Antiochus IV Epiphanes and Laodice IV and heir to the Seleucid throne. The ancient sources, Polybius and Diodorus say that this claim was false and that he and his sister Laodice VI were really natives of Smyrna of humble origin. However, Polybius became friends with Balas's rival King Demetrius I when both were hostages in Rome, so Polybius is not an unbiased source on this matter. Modern scholars disagree about whether the story of Attalus finding a commoner who looked the part is true or was propaganda put about by Alexander's opponents.

According to Diodorus, Alexander was originally put forward as a candidate for the Seleucid throne by Attalus II of Pergamum. Attalus had been disturbed by the Seleucid king Demetrius I's interference in Cappadocia, where he had dethroned king Ariarathes V. Boris Chrubasik is sceptical, noting that there is little subsequent evidence for Attalid involvement with Alexander. However, Selene Psoma has proposed that a large set of coins minted in a number of cities under Attalid control in this period was produced by Attalus II in order to fund Alexander's bid for the kingship.

Alexander and his sister were maintained in Cilicia by Heracleides, a former minister of Antiochus IV and brother of Timarchus, an usurper in Media who had been executed by the reigning king Demetrius I Soter. In 153 BC, Heracleides brought Alexander and his sister to Rome, where he presented Alexander to the Roman Senate, which recognised him as the legitimate Seleucid king and agreed to support him in his bid to take the throne. Polybius mentions that Attalus II and Demetrius I also met with the Senate at this time but does not state how this was connected to the recognition of Alexander - if at all.

After recruiting mercenaries, Alexander and Heracleides departed to Ephesus. From there, they invaded Phoenicia by sea, seizing Ptolemais Akko. Numismatic evidence shows that Alexander had also gained control of Seleucia Pieria, Byblos, Beirut and Tyre by 151 BC. On this coinage, Alexander heavily advertised his (claimed) connection to Antiochus IV, depicting Zeus Nicephorus on his coinage as Antiochus had done. He also assumed the title of Theopator ('Divinely Fathered'), which recalled Antiochus' epithet Theos Epiphanes ('God Manifest'). The coinage also presented Alexander Balas in the guise of Alexander the Great, with pronounced facial features and long flowing hair. This was intended to emphasise his military prowess to his soldiers.

Alexander and Demetrius I competed with another to win over Jonathan Apphus, the leader of the ascendant faction in Judaea. Jonathan was won over to Alexander's side by the grant of a high position in the Seleucid court and the high priesthood in Jerusalem. Reinforced by Jonathan's hardened soldiers, Alexander fought a decisive battle with Demetrius in July 150 BC, in which Demetrius was killed. By autumn, Alexander's kingship was recognised throughout the Seleucid realm.

Alexander gained control of Antioch at this time and his chancellor, Ammonius, murdered all the courtiers of Demetrius I, as well as his wife Laodice and his eldest son Antigonus. Ptolemy VI Philometor of Egypt entered into an alliance with Alexander, which was sealed by Alexander's marriage to his daughter Cleopatra Thea. The wedding took place at Ptolemais, with Ptolemy VI and Jonathan Apphus in attendance. Alexander took the opportunity to shower honours on Jonathan, whom he treated as his main agent in Judaea. The marriage was advertised by a special coinage issue, depicting the royal pair side by side - only the second depiction of a queen on Seleucid coinage. She is shown with divine attributes (a cornucopia and a calathus) and is depicted in front of the king. Some scholars have seen Alexander as little more than a Ptolemaic puppet, arguing that this coinage emphasises Cleopatra's dominance over him and that the chancellor Ammonius was a Ptolemaic agent. Other scholars argue that the alliance was advertised as an important one, but that the arguments for Alexander's subservience have been overstated.

Meanwhile, the Seleucid positions in the eastern Upper Satrapies, already weakened by the previous kings' failure to contain the Parthians and the Greco-Bactrians, suffered almost complete collapse. The Parthians under Mithridates I took advantage of the general instability to invade Media. The region had been lost to Seleucid control by the middle of 148 BC. At around the same time the local nobles in Elymais and Persis asserted their own ephemeral independence, only to be soon also subdued by the Parthians. By 148 BC at the latest the Parthians also secured their hold over Hyrcania at the coast of the Caspian Sea. By 147 BC the Parthians stood at the doorsteps of Babylonia, one of the Seleucid empire's hearthlands and location of one of its two capital cities, Seleucia-on-Tigris.