Tanchelm (died 1115), also known as Tanchelm of Antwerp, Tanchelijn or Tanchelin, was a heretical itinerant preacher, critical of the established Roman Catholic church, active in the Low Countries around the turn of the 11th and 12th centuries.
Tanchelm, the date and place of whose birth is unknown, was supposed to have been a monk, perhaps from the circle of Count Robert II of Flanders (1092–1111). From 1112 he preached in Antwerp, the Duchy of Brabant, Flanders and Zeeland against the official church and its hierarchy, opposed the payment of tithes and those priests who lived with women. He was apparently also in Rome, where he is supposed to have campaigned, in vain, for an extension of the Bishopric of Thérouanne to cover the islands of the Scheldt. He was briefly put under arrest in Cologne in 1113/1114 but released again, despite the vigorous protests of the cathedral clergy of Utrecht. In 1115, after he had seized Antwerp and declared himself king, a priest bashed his head in and killed him while floating downriver on the royal barge.
The followers of Tanchelm, who is reported to have allowed himself to be venerated almost to the point of worship, were still to be found for a period after his death in Antwerp; in 1124 Saint Norbert of Xanten preached against their heresies.