Host desecration is a form of sacrilege in Christian denominations that follow the doctrine of the real presence of Christ in the Eucharist. It involves the mistreatment or malicious use of a consecrated host—the bread used in the Eucharistic service of the Divine Liturgy or Mass (also known by Protestants simply as Communion bread). It is forbidden by the Catholic, Oriental Orthodox, and Eastern Orthodox Churches, as well as in certain Protestant traditions (including Anglicanism, Lutheranism, and Methodism). In Catholicism, where the host is held to have been transubstantiated into the body of Jesus Christ, host desecration is one of the gravest sins. Intentional host desecration incurs the penalty of excommunication latae sententiae. Throughout history, a number of groups have been accused of desecrating the Eucharist, often with grave consequences due to the spiritual importance of the consecrated host.

Accusations against Jews were a common reason given for massacres and expulsions throughout the Middle Ages in Europe. Similar accusations were made in witchcraft trials; witch-hunter's guides such as the Malleus Maleficarum refer to hosts as being objects of desecration by witches. It is part of many descriptions of the Black Mass, both in ostensibly historical works and in fiction.