Islamic sources from the seventh to eleventh centuries refer to Berbers, Vikings, Qadariyya, and Hindus as “Zoroastrians” (majūs). Previous scholarship attempted to explain this curious practice through some purported link between these groups and actual Zoroastrians. Such links are tenuous at best. Other scholars interpreted the term as a legal category for non-Muslims who did not meet the Qur’anic standard of People of the Book (ahl al-kitāb). However, that does not explain its use with regard to Muslims or groups that remained beyond the reach of Islamic law. This paper argues that Muslims used the term “Zoroastrians” rhetorically, applying it toward any group deemed unfamiliar, pagan, or heretical. When Viking raiders harried the coasts of Spain in the ninth century, Andalusi Muslims called them “Zoroastrians” because they were unfamiliar. Andalusi Muslims continued to identify Vikings primarily as Zoroastrians even after adopting more accurate terminology, like Urdumāniyyūn (Northmen). Yet Muslims from further east, like Ahmad ibn Fadlan, did not use this misnomer because they had experience with Zoroastrians from Iran. In Medina, early Islamic philosophers dismissed their antagonists as “Zoroastrians.” The Jabriyya, proponents of predestination, deemed the Qadariyya, proponents of free will, to be such, although not because their philosophy bore a resemblance to Zoroastrian beliefs. Rather, according to heresiographical literature like ‘Abd al-Qadir al-Baghdadi’s Farq bayn al-Firāq, Jabriyya should limit their association with the heretical Qadariyya in the same way that Muslims were supposed to limit their association with Zoroastrians. The comparison was figurative, not literal. Shi‘i Muslims also adopted this term. In the tenth century, the Fatimid caliph al-Mu‘izz removed an Isma‘ili missionary (dā‘ī) from his post in India after the man allowed recent converts to Islam to observe their previous dietary customs, which the court found objectionable. The Fatimid jurist al-Nu‘man, in a letter to his subordinates in Sind, labeled these lapsed Muslims “Zoroastrians” in order to emphasize their heterodoxy. Indeed, elsewhere in the text he simply calls them mushrikūn. In each of these cases—drawn from across the early Islamic world—the best interpretation of the available evidence is that Muslims applied the term “Zoroastrians” to groups that were unfamiliar, pagan, or heretical. All were rhetorical Zoroastrians; these groups had no plausible connection to Iranian Zoroastrianism. Therefore, this nomenclature sheds light on the process of identity formation among Muslims, as well as lingering ambivalence about Zoroastrians, in early Islamic history.