An ergastula was a Roman building used to hold in chains dangerous slaves, or to punish other slaves. The ergastula was usually subsurface, built as a deep, roofed pit - large enough to allow the slaves to work within it and containing narrow spaces in which they slept. They were common structures on all slave-using farms (latifundium). The etymology is disputed between two possible Greek roots 'ergasterios' (workshop) and 'ergastylos' (pillar to which slaves were tethered).

The ergastula was made illegal during the reign of Hadrian.

The term is also used to describe any small Roman prison.