A theatre at Landmark Cinemas in Kitchener was cleared out on Friday afternoon after reports that a "noxious substance" was released.
"The effected theatre has been cleared and we are now on scene with other emergency services," said Constable Andre Johnson with Waterloo Regional Police.
He says the substance was not released in the theatre that was airing the new Joker movie.
At the time of the incident, a number of other movies were playing including Abominable, Downtown Abbey and War.
"We are still investigating to determine exactly what occurred, what was sprayed and to find the person that was responsible," added Johnson.
Anyone with information is asked to call police or Crime Stoppers.