Egon Spengler, PhD is a fictional character from the Ghostbusters franchise. He appears in the films Ghostbusters, Ghostbusters II, and Ghostbusters: Afterlife, in the animated television series The Real Ghostbusters and Extreme Ghostbusters, and in the video games Ghostbusters: The Video Game and Ghostbusters Beeline. Egon was portrayed by Harold Ramis in the films and voiced by him in Ghostbusters: The Video Game and Lego Dimensions, and voiced by Maurice LaMarche in the cartoon series. He is a member of the Ghostbusters and one of the three doctors of parapsychology, along with Dr. Peter Venkman and Dr. Ray Stantz.

The character's name, Egon Spengler, is an amalgamation of the name Egon Donsbach, who was a Hungarian refugee classmate of Ramis' at Senn High School, and the name of German polymath Oswald Spengler.

Christopher Walken, John Lithgow, Christopher Lloyd, and Jeff Goldblum were all considered for the role of Egon Spengler, before Ramis, who had gotten close to the character while writing him, felt compelled to play it himself.

Maurice LaMarche stated that when he auditioned for the part of Egon in The Real Ghostbusters, he was asked not to do an impression of Ramis, which he ignored because impressions were one of his strengths as a performer, and there was no other way he could imagine properly portraying the character other than to follow Ramis' example. He got the part anyway and said in an interview that he did two different takes: one in which he impersonated Ramis, the other in which he tried a more "Woody Allen"–like approach that did not suit the character's physicality.

Egon Spengler is a tall, lanky, laconic, bespectacled, handsome member of the team responsible for the main theoretical framework for their paranormal/quantum studies, having earned over a dozen advanced degrees including parapsychology and nuclear engineering from New York University and Massachusetts Institute of Technology, respectively. He is socially awkward, as demonstrated by his stiff interactions with the Ghostbusters' secretary Janine Melnitz, and his reliance on Peter Venkman as a spokesperson for the group.

Egon is the most serious and rigid member of the team. Of his hobbies, Egon states that he collects "spores, molds, and fungus", and claims that, as a child, the only toy he ever had was "part of a Slinky", which he straightened out. As implied in the first movie, Egon apparently is a junk food junkie, due to his affection for sweets and snack crackers. In the second film it is revealed that he and Ray like takeout foods like pizza, Asian, Greek, and Mexican cuisine; in the first film, there is also a scene where Egon dines on Chinese takeout with Peter and Ray, during which he eats egg rolls while he finishes building the proton packs. Despite being a stereotypical timid professor-like figure, Egon is prone to violent reactions when pushed too far temperamentally, as demonstrated by his attempt to assault Walter Peck.

After Egon dies, the haunting in his farmhouse reveals that, like Ray, he was a skilled automotive mechanic, able to repair the Ecto-1's engine with his grandson Trevor as a poltergeist.

Egon was very interested in paranormal phenomena, even while working with Ray Stantz and Peter Venkman at Columbia University. He and Ray Stantz studied paranormal literature in their spare time, and Egon personally follows John Horace Tobin's works including Tobin's Spirit Guide, and were interested in theories of reincarnation. It is mentioned in the film's novelization that Egon's mother is alive, and he calls her after finding a payphone to inform his termination of his employment at Columbia University, while Peter and Ray discussing plans for the Ghostbusters business, explaining his absence on this scene.