A juggernaut (/ˈdʒʌɡənɔːt/ ^ⓘ), in current English usage, is a literal or metaphorical force regarded as merciless, destructive, and unstoppable. The term frequently implies an out-of-control force or object.

This English usage originates in the mid-nineteenth century. Juggernaut is the early rendering in English of Jagannath, an important deity in the Hindu traditions of eastern and north-eastern India. The meaning originates from the Hindu temple cars, which are chariots, often huge, used in processions or religious parades for Jagannath and other deities, the largest of which, once set into motion, are difficult to stop, steer or control by humans, on account of their massive weight.

Since the Middle Ages, Europeans had been fascinated by accounts of the Ratha Yatra (lit. 'temple car procession') at Puri, which claimed that pilgrims threw themselves under the temple cars. However, by 1825 it was said: "That excess of fanaticism which formerly prompted the pilgrims to court death by throwing themselves in crowds under the wheels of the car of Jaganath, has happily long ceased." Despite this, a New York Times report from 1864 reported witnessing celebrants deliberately placing themselves under the wheels of the device against the will of the authorities.

The figurative use of the word is analogous to figurative uses of steamroller or battering ram to mean something overwhelming. Its ground in social behavior is similar to that of bandwagon, but with overtones of devotional sacrifice. Its British English meaning of a large heavy truck or articulated lorry dates from the second half of the twentieth century.

The word is derived from the Sanskrit/Odia Jagannātha (Devanagari जगन्नाथ, Odia ଜଗନ୍ନାଥ) "world-lord", combining jagat ("world") and nātha ("lord"), which is one of the names of Krishna found in the Sanskrit epics.

The English loanword juggernaut in the sense of "a huge wagon bearing an image of a Hindu god" is from the seventeenth century, inspired by the Jagannatha Temple in Puri, Odisha (Orissa), which has the Ratha Yatra ("Temple car procession"), an annual procession of chariots carrying the murtis (images) of Jagannātha, Subhadra, and Balabhadra.

The first European description of this festival is found in a thirteenth-century account by the Late Medieval Franciscan friar and missionary Odoric of Pordenone, who describes Hindus, as a religious sacrifice, casting themselves under the wheels of these huge chariots and being crushed to death. Odoric's description was later taken up and elaborated upon in the popular fourteenth-century Travels of John Mandeville. Others have suggested more prosaically that the deaths, if any, were accidental and caused by the press of the crowd and the general commotion. Contemporaneous reports from colonial Kolkata report witnessing intentional suicides at the processions which were either tacitly allowed or else ignored by clerics, despite the practice being prohibited by government policy.

Many speakers and writers apply the term to a large machine, or collectively to a team or group of people working together (such as a highly successful sports team or corporation), or even a growing political movement led by a charismatic leader—and it often bears an association with being crushingly destructive towards all obstacles.