Johannes Wier (Latin: Ioannes Wierus or Piscinarius; 1515 – 24 February 1588) was a Dutch physician who was among the first to publish a thorough treatise against the trials and persecution of people accused of witchcraft.

The heirs of the dead and diseased had actually paid people at Casale to smear the gates in order to obtain their inheritances more quickly.Weyer criticised the Malleus Maleficarum and the witch hunting by the Christian and Civil authorities; he is said to have been the first person that used the term mentally ill or melancholy to designate those women accused of practicing witchcraft.

[citation needed] Nevertheless, while he defended the idea that the Devil's power was not as strong as claimed by the orthodox Christian churches in De Praestigiis Daemonum, he defended also the idea that demons did have power and could appear before people who called upon them, creating illusions; but he commonly referred to magicians and not to witches when speaking about people who could create illusions, saying they were heretics who were using the Devil's power to do it, and when speaking on witches, he used the term mentally ill.[3] Moreover, Weyer did not only write the catalogue of demons Pseudomonarchia Daemonum, but also gave their description and the conjurations to invoke them in the appropriate hour and in the name of God and the Trinity, not to create illusions but to oblige them to do the conjurer's will, as well as advice on how to avoid certain perils and tricks if the demon was reluctant to do what he was commanded or a liar.

In addition, he wanted to abolish the prosecution of witches, and when speaking on those who invoke demons (which he called spirits) he carefully used the word exorcist.

There were many editions of his books (written in Latin), especially Pseudomonarchia Daemonum, and several adaptations in English, including Reginald Scot's "Discoverie of Witchcraft" (1584).

Weyer's appeal for clemency for those accused of the crime of witchcraft was opposed later in the sixteenth century by the Swiss physician Thomas Erastus, the French legal theorist Jean Bodin and King James VI of Scotland.

Johan was the son of Agnes Rhordam and Theodorus (Dirk) Wier, a merchant of hops, coal and slate, who was a schepen of Grave in the 1520s.

Dirk and Agnes Wier came from Zeeland and were closely acquainted with Maximiliaan van Egmond and Françoise de Lannoy, the future in-laws of William the Silent.

Johan's oldest son, Diederik Wier, became a jurist and diplomat, who in 1566-7, while employed by Willem IV van den Bergh, was involved in the "petitions of grievances about the suppression of heresy" by the Dutch nobility to Philip II of Spain, the rejection of which led to the Eighty Years' War.