In folklore, a will-o'-the-wisp, will-o'-wisp or ignis fatuus (Latin for 'giddy flame'), plural ignes fatui), is an atmospheric ghost light seen by travellers at night, especially over bogs, swamps or marshes. The phenomenon is known in English folk belief, English folklore and much of European folklore by a variety of names, including jack-o'-lantern, friar's lantern, and hinkypunk and is said to mislead travellers by resembling a flickering lamp or lantern. In literature, will-o'-the-wisp metaphorically refers to a hope or goal that leads one on, but is impossible to reach, or something one finds strange or sinister.

Wills-o'-the-wisp appear in folk tales and traditional legends of numerous countries and cultures; notable wills-o'-the-wisp include St. Louis Light in Saskatchewan, the Spooklight in Southwestern Missouri and Northeastern Oklahoma, the Marfa lights of Texas, the Naga fireballs on the Mekong in Thailand, the Paulding Light in Upper Peninsula of Michigan and the Hessdalen light in Norway.

In urban legends, folklore and superstition, wills-o'-the-wisp are typically attributed to ghosts, fairies or elemental spirits. Modern science explains the light aspect as natural phenomena such as bioluminescence or chemiluminescence, caused by the oxidation of phosphine (PH₃), diphosphane (P₂H₄) and methane (CH₄) produced by organic decay.