Filming took place at the Corvin Castle in Hunedoara, and in Sighisoara, Transylvania, Romania, and also at Mogosoaia Palace, near Bucharest, Romania.
Taissa Farmiga, who plays Sister Irene in this film, is the younger sister of Vera Farmiga, who played Lorraine Warren in Invocação do Mal (2013), Invocação do Mal 2 (2016), and Invocação do Mal 3: A Ordem do Demônio (2021). Though the sisters' characters are not at all related to each other, they are both devout Catholics with clairvoyant abilities. The Demon Nun, played by Bonnie Aarons, also appeared in Invocação do Mal 2 (2016) and had a brief cameo in Annabelle 2: A Criação do Mal (2017).
While Invocação do Mal (2013) and Invocação do Mal 2 (2016) are based on allegedly true events, this film's story is entirely made up by the filmmakers.
After the "Demon Nun" from Invocação do Mal 2 (2016) proved to be a popular horror antagonist, a spin-off focusing on the character was green-lit, making her the second character to get her own feature after Annabelle (2014). The Crooked Man, from Conjuring 2, was also to get his own spin-off, which was later cancelled.
The events take place before Annabelle 2: A Criação do Mal (2017), making it the first film (chronologically) in the series. Although the opening scenes of Annabelle: Creation take place before The Nun, the rest occurs afterward.