Short answer: Catapults were first developed by ancient Greek engineers under Dionysius I of Syracuse around 400–399 BCE, evolving from powerful bow-like artillery; later forms spread across the Hellenistic and Roman worlds and into medieval siege warfare.

Greek origins

Ancient sources and modern histories attribute the first mechanical artillery (early catapults) to a Syracusan weapons program under Dionysius the Elder circa 400–399 BCE, initially producing arrow‑shooting machines and then stone‑hurlers. Greek accounts note these were conceived by soldiers/engineers seeking longer‑range strike capability, reportedly fielded soon after in Sicilian campaigns.

Early types

Two branches emerged from the Greek innovations: the double‑armed ballista for bolts and the single‑armed stone thrower, both relying on torsion bundles; Romans later standardized and mobilized these designs with wheeled carriages. The onager/mangonel style single‑arm torsion thrower is often associated with Roman and later medieval use, though its precise naming and evolution are clarified in late antique and medieval sources.

Broader claims

General references often summarize that “catapults were invented by the ancient Greeks,” reflecting the 5th–4th century BCE origin point; some summaries also note early Indian uses, but the Greek program under Dionysius is the most specifically dated and documented in classical military history. Scholarly surveys and reconstructions consistently place the decisive invention phase in late 5th to early 4th century BCE Greek engineering circles.