Gaius Helen Mohiam is a fictional character in the Dune universe created by Frank Herbert. She is the Bene Gesserit Reverend Mother, who initially featured in the novel Dune by Frank Herbert. She also had a more major role in the Prelude to Dune trilogy by Brian Herbert and Kevin J. Anderson.

Mohiam trained Lady Jessica in the Bene Gesserit chapterhouse on Wallach. Two decades later, Mohiam tested the young Paul Atreides before his departure for Arrakis with the gom jabbar. The test was to discover whether Paul was "truly human" by trial of pain. The pain inflicted when Paul put his hand in the box was caused by nerve induction and clearly drains an incredible amount of energy from the Reverend Mother.

In the novel Dune Messiah, the Reverend Mother joined the conspiracy to topple the rule Paul Atreides after he ascended the Lion Throne and unleashed his Jihad upon the Empire. She, along with Scytale (a Tleilaxu face dancer), Edric (a Spacing Guild navigator), and Irulan (embittered wife of Paul Atreides), plot to insinuate the Tleilaxu ghola Hayt into the court to weaken Paul's confidence in his prescience and thereby ruin him. When the plot failed, she was executed in the retailiation initiated by Alia Atreides after Paul walked into the desert after the death of his wife Chani.

The prequel novels by Brian Herbert and Kevin J. Anderson that began with Dune: House Atreides and Dune: House Harkonnen reveal that Mohiam was secretly the mother of Jessica and grandmother of Paul, in contradiction to Children of Dune, which states Jessica's mother as Tanidia Nerus. This occurred as a result of a vicious rape by the Baron Harkonnen. Many hold the information in these prequels to be apocryphal, although the writers Brian Herbert and Kevin J. Anderson may have been working from notes by Frank Herbert himself on events concerning Mohiam.