I study how humans related to and understood aquatic nature in the early modern period through three artistic materials extracted from the sea: coral, shells, and marine fossils. Rather than passive commodities, these objects speak to period notions about the transformative power of nature. Each substance advanced theories in natural philosophy, strengthened civic identities, heightened the power of devotional objects, and reinforced connections between water and the marvelous. Galatea’s Realm traces the cultural importance of the sea through three case studies: the imperial exploitation of Sicilian coral art, Neapolitan still-life paintings of underwater curiosities, and the development of paleontology through scientific illustrations of marine fossils. Drawing from ecomaterialism and the history of science, each chapter demonstrates how marine materials played an active role in shaping religion, science, and geopolitics during the seventeenth century.