According to Hesiod’s Theogony, Chaos was the first primordial deity to exist.
In Theogony 116–133, Hesiod describes Chaos as the first being, followed by Gaia (Earth), Tartarus, and Eros. Here, “Chaos” does not primarily mean confusion or disorder. The Greek word suggests a chasm, gap, or yawning space—the primordial condition from which the divine genealogy begins.
Chaos then produced Erebus (Darkness) and Nyx (Night). Their descendants included Aether (the bright upper air) and Hemera (Day), while Gaia independently brought forth Ouranos (Sky), the mountains, and Pontus (Sea). These beings represent fundamental cosmic forces rather than later, human-like Olympian gods.
A common mix-up is choosing Gaia, because she is the first major earth power and the mother of many divine generations. However, Hesiod explicitly places Chaos first. Other Greek creation traditions, especially Orphic accounts, arrange the first beings differently, but the question specifically names Hesiod’s Theogony.