The text of the book entitled On Rivers, attributed to Pseudo-Plutarch, is based on the manuscript Palatinus Heidelbergensis gr. 398 (Codex Palatinus Graecus 398, fols. 157r-173r), dated to the ninth century CE and housed in the Heidelberg University Library. Since the author of the text is identified as Plutarch, it was initially assumed that the book belonged to the famous Mestrius Plutarch (ca. 46-120 CE). However, when the author’s vocabulary, intellectual level, and the references employed in the composition of the text are considered, it becomes clear that On Rivers cannot be attributed to Mestrius Plutarch but rather to another author bearing the same name. For this reason, the author has come to be referred to as Pseudo-Plutarch. The book addresses twenty-five significant rivers from various regions of the known world and takes the form of a catalog providing geographical, onomastic, etymological, mythological, and paradoxographical information about these rivers. While some modern scholars dismissed Pseudo-Plutarch’s On Rivers as unreliable and therefore negligible, others used it as a reference source to substantiate their arguments. The present article was written in order to compare the narrations about the Euphrates (Εὐφράτης = Fırat) and the Tigris (Τίγρις = Dicle) found in On Rivers, attributed to Pseudo-Plutarch, with those of other ancient authors, and to question the accuracy of Pseudo-Plutarch’s narrations regarding these two rivers. The geographical, onomastic, etymological, mythological, and paradoxographical information provided in the book is compared to the writings of other ancient authors as well as with modern scholarship, and its reliability is critically examined. It is therefore recommended that scholars exercise caution and adopt a critical perspective when citing Pseudo-Plutarch’s On Rivers as a reference.