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 The Conjuring (2013), The Conjuring 2 (2016), and The Conjuring: The Devil Made Me Do It (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 The Conjuring 2 (2016) and had a brief cameo in Annabelle: Creation (2017).
After the "Demon Nun" from The Conjuring 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.
While The Conjuring (2013) and The Conjuring 2 (2016) are based on allegedly true events, this film's story is entirely made up by the filmmakers.
The events take place before Annabelle: Creation (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.