The Brussels massacre was an anti-Semitic episode in Brussels (then within the Duchy of Brabant) in 1370 in connection with an alleged host desecration at the Brussels synagogue. In May 22, a number of Jews, variously given as six or about twenty, were executed or otherwise killed, while the rest of the small community was banished.

The supposedly recovered hosts became objects of veneration for local Christians as the Sacrament of Miracle. The cult survived until after the Second World War and the Holocaust, after which its antisemitic elements pushed the local church to derecognise it. It was claimed that the hosts had been stabbed by Jews and had miraculously shed blood, but been otherwise unharmed. The reliquary (without the hosts) is currently preserved in the cathedral's treasury (the hosts may have been present inside it as late as the year 2000).

Black Death Jewish persecutions had previously destroyed Brussels' Jewish community in 1350. According to Premonstratensian historian Placide Lefèvre, contemporary treasury records indicate that by 1370 there were eight Jewish households in Brussels and two in Leuven.

In 1369, two priests in Brussels were arrested for usury and turned over to the ecclesiastical tribunal of the Church of St. Michael and St. Gudula (now the cathedral) for investigation: it transpired that they had attempted to circumvent usury prohibitions by lending money to a Jew named Mesterman who in turn lent it out at interest. The clerical usury scandal in Brussels was the immediate context of the accusations of host desecration, a common anti-Semitic canard in medieval Europe, with wafers that Jews had supposedly tried to profane often said to have been miraculously spared from harm.

The version of the allegations attested from 1403 was that a rich Jew from Enghien wanted to obtain some consecrated hosts, also known as Communion bread or a sacred host, to profane, and bribed a male Jewish convert to Christianity from Leuven to steal some. Shortly thereafter, the Enghien merchant was murdered. His widow passed the stolen hosts to the Jews of Brussels, where in the synagogue on Good Friday 1370 some tried to stab the wafers with their daggers, causing blood to pour forth. A female Jewish convert to Christianity was paid to take the hosts to Cologne's Jews, but remorsefully told the story to the parson of Notre-Dame de la Chapelle in Brussels, who took possession of the hosts. The Duke of Brabant, on the woman's testimony, ordered the stabbers burnt at the stake and the remaining Jews banished, with their property confiscated.

The hosts were placed in reliquaries and preserved in the then collegiate church of Saint Gudula, the patron saint of Brussels, an important symbol of the area's Catholic identity. They became a feature of the annual procession on her feast day.

Emperor Charles V and the Habsburgs, as well as their relatives, donated seven stained glass windows on which the miracle is depicted. These were executed by Antwerp glass-maker Jan Hack after creations by Bernard van Orley and Michiel Coxcie. Four of them are still in place today. This compared perceived Jewish anti-Catholicism to the nascent Protestant Reformation, with the miraculous bleeding countering Protestant denials of transubstantiation.

In the early 1580s, during a period of Calvinist rule in Brussels, all Catholic ceremonies were suppressed. From 1579 to 1585 the relics had been hidden in a house in the Korte Ridderstraat. After the end of Calvinist rule in 1585, a procession of citizens and officeholders had retrieved the hosts and carried them back to the church. The re-emergence of the cult in 1585 was primarily as a celebration of the end of Calvinist rule. The Archdukes Albert and Isabella, who ruled in Brussels 1598–1621, made the annual procession a state occasion: