Trial by ordeal is a judicial practice by which the guilt or innocence of the accused is determined by subjecting them to a painful task. If either the task is completed without injury, or the injuries sustained are healed quickly, the accused is considered innocent. In medieval Europe, like trial by combat, it was considered a judicium Dei: a procedure based on the premise that God would help the innocent by performing a miracle on their behalf. The practice has much earlier roots however, being attested in polytheistic cultures as far back as the Code of Hammurabi and the Code of Ur-Nammu, and in animist tribal societies, such as the trial by ingestion of "red water" (calabar bean) in Sierra Leone, where the intended effect is magical rather than invocation of a deity's justice.

In pre-modern society, the ordeal typically ranked along with the oath and witness accounts as the central means by which to reach a judicial verdict. Indeed, the term ordeal itself, Old English ordel, has the meaning of "judgement, verdict" (German Urteil, Dutch oordeel), from Proto-Germanic *uzdailjam "that which is dealt out".

In Europe, ordeals commonly required an accused person to test himself or herself against fire or water, though the precise nature of the proof varied considerably at different times and places. In England, ordeals were common under both the Anglo-Saxons and the Normans. Fire was the element typically used to test noble defendants, while water was more commonly used by lesser folk.

Priestly cooperation in trials by fire and water was forbidden by Pope Innocent III at the Fourth Lateran Council of 1215, and replaced by compurgation. . Trials by ordeal became more rare over the Late Middle Ages, often replaced by confessions extracted under torture, but the practice was discontinued only in the 16th century. Johannes Hartlieb in 1456 reports a popular superstition on how to identify a thief by an ordeal by ingestion practiced privately without judicial sanction.