This article explores how the planet Saturn was conceptualized in the premodern cosmology that influenced the Florentine Renaissance philosopher Marsilio Ficino (1433–1499), particularly in the Latin translations of Arabic astrological works from which he drew inspiration in the production of his medical guide and mirror for princes, the De vita libri tres (1489). It puts special attention on the secret role of the astro-magical treatise Picatrix in Ficino’s intellectual development and on some of the astrological correspondences or significations between Saturn and the concepts over which that planet was believed to hold dominion (e.g., melancholy, scholarship, and others). The article traces out some of the changes in the perceptions of Saturn, from his ambivalent role as a chthonic Roman divinity and the ruler of an agricultural golden age to a largely malefic force of nature in Late Antiquity up to Abū Maʿšar, and returning again to an ambivalent role as a volitional spiritual entity and a ruler of duality and extremes in the minds of astrologers from al-Qurṭubī (d. 964) to Ficino, who were also influenced by pseudo-Aristotelian, Aristotelian, Hermetic, and other pagan sources in constructing their knowledge of astral magic.