This dissertation investigates the co-reign and reign of the second king of the Seleucid Empire, Antiochus I Soter (ca. 294–261 BCE) and serves as the first extensive study of his reign. I argue that the reign of Antiochus I is the period of Seleucid history in which we can identify the first coherent formation of Seleucid imperial ideology and of a Seleucid dynastic identity. To make this argument, I draw on a wide range of evidentiary sources: literature, archaeology, numismatics, epigraphy, and other material sources. Antiochus implemented for the first time or in new ways many facets of Seleucid rule that would come to define the dynasty’s imperial approach for nearly all subsequent Seleucid kings. Among Antiochus’ innovations were the creation of the Seleucid Era, the elevated importance of Seleucid Syria, a defined ideology of Seleucid territory and its bounds, and the introduction of Apollo as a Seleucid dynastic ancestor. The reign of Antiochus has long been overlooked in modern scholarship, and many of these new policies have often been attributed to Antiochus’ father Seleucus. I argue that this retroactive understanding of these innovations was the intent of Antiochus’ ideological program. In the years after Seleucus’ death in 281 BCE, Antiochus created a dynastic identity which was centered around a newly constructed history of Seleucus’ life and reign. In forging an imaginary history of Seleucus’ life and achievements, Antiochus solidified a coherent dynastic identity which simultaneously strengthened and deemphasized his own reign by rooting it in a dynastic past and which could also be used as a tool to justify new imperial action. As Antiochus introduced his new Seleucid policies, he downplayed their novelty by inserting them into this new Seleucid past, presenting them as the continuation of things that had already been done and which already existed.