The author reveals basic aspects of Xenophanes of Colophon’s natural-philosophical monism as attested in the writings of ancient authors, especially in those of Aristotle, Cicero and Sextus Empiricus, and compares his concept of a “single”, “eternal”, “infinite” (see: Cicero. On the Nature of the Gods, I, 11, 28) and “rational” god/the Universe (“nature”) with a number of key aspects of Spinoza’s panentheistic doctrine of the single substance – “God, or Nature” (the Universe). Since, on the one hand, Spinoza’s personal library included Aristotle’s works translated into Latin and the Thesaurus Ciceronianus containing, in particular, the passages on Xenophanes’s doctrine from Cicero’s treatises, and, on the other hand, some printed editions of the Latin translations of Sextus Empiricus’s works were widely available in his time, one can assume that Spinoza could be familiar with Xenophanes’s natural-philosophical conception.