The film, starring Joaquin Phoenix as Batman's sadistic nemesis, has proved controversial for its portrayal of violence and there have been heightened security fears surrounding its release.
Police in cities across the US, including New York and Las Vegas, have stepped up patrols at cinemas showing the movie.
Family members of victims of a 2012 mass shooting by a man reportedly identifying himself as Joker have expressed concerns about the film.
On Friday, it emerged a cinema in Huntington Beach, a city south east of Los Angeles, had cancelled two Thursday night showings of the Joker after police were called.
The Huntington Beach Police Department said it had received reports of a possible threat to the Century Theatre at the Bella Terra shopping centre and were treating it as “credible”.
The cinema reopened on Friday, police said. They declined to provide details of the threat.
Joker is directed by Todd Phillips and acts as an origins story for Joker. Set in 1981, it follows failed stand-up comedian Arthur Fleck, who turns to a life of crime in Gotham City.
Warner Bros, the studio behind the film, has defended it, saying “neither the fictional character Joker, nor the film, is an endorsement of real-world violence of any kind”.
The film, rated a 15 in the UK for “strong bloody violence”, also features Robert De Niro, Zazie Beetz and Frances Conroy in supporting roles.
The Joker is in UK theatres now.
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies