WARNING: THE FOLLOWING ARTICLE CONTAINS SPOILERS! READ AT YOUR OWN RISK!
- #1: Killer Croc: In Intensive Treatment, you can see an area with security collars like the one worn by Killer croc and photos of him; there's also a locker with a photo of Croc's hand with a message saying "Hands off my locker". Also, near that section you can see a where the Asylum staff leaves meat for Croc.
- #2: "You don't have to be mad to work at this Asylum!": In the Secure Transit of Intensive Treatment you can find a glass saying "You don't have to be mad to work here... But it certainly helps."
- #3: Ra's al Ghul: After discovering Ra's al Ghul's body bag in the Arkham Mansion, if the player goes back to the Morgue it has mysteriously disappeared.
- #4: Visitor Center: When Batman enters the Visitor Center, if the player turns around for some time the mannequin's arms have changed, and near the end of the game it's revealed that the mannequin is actually Joker himself.
- #5: Rocksteady: In most locations the player can notice the pictures of the supposed patients are actually photos Rocksteady Studios' crew. A thug holding Carl Todd hostage will say "You see Hego over there? He's got a thing for blades." David Hego is Rocksteady's art director.
- #6: Paging Dr. Clooney: After you rescue Aaron Cash and Dr. Kellerman from Joker's laughing gas, stand in the main room looking at the smaller room where they were trapped. A ticker scrolls a garbled message: Dr. Clooney report to Gynecology." It is a reference to George Clooney, who played Batman in Michael Keaton's Batman and Robin.
- #7: Drs. Murphy and Combs: The first Scarecrow Interview Tape features the names "Dr. Murphy" and "Dr. Combs". This is a reference to Cillian Murphy, who portrayed Scarecrow in Christopher Nolan's Dark Knight trilogy and Jeffrey Combs, who voiced Scarecrow in The New Batman Adventures.
- #8: The Robins: Jason Todd and Tim Drake, second and third to take the mantle of Robin, are referred to by Joker in one of his Visitor Center messages where he says: "What's it like in your organization? Do you punish your hired help when they fail? Or is that saucy outfit you make him wear punishment enough?" Also, there are scribbles on a few walls throughout the asylum depicting what appears to be a reference to the original Red Hood and abandoned wheelchairs.
- #9: Blueprints: In Batman: Arkham City, during the first Catwoman mission there is a side table with a map and blueprints on it. The blueprints are also shown in the Cyrus Pinkney Institute for Natural History as well as the Gotham City Police Department. This blueprint is also found in the Warden's secret room and Intensive Treatment.
- #10: Alternate Endings: After you beat the game and the credits come to an end you will see one of three ending scenes. They seem to be random and not based on completion percentage: As the Batwing flies towards Gotham Bane, Killer Croc or Scarecrow's arm rises out of the water near the asylum docks and grabs a floating crate of Titan.
- #11: ArkhamCare: As you walk through the halls of the Medical Facility, there's a voice in the speakers that will read a website: http://www.arkhamcare.com. Although the site has expired, there are still archives of it.
- #12: Hugo Strange: In the Arkham Mansion where Cash is after the completion of the game, there's a statue similar to Hugo Strange, implying he was behind everything in the asylum and onwards.
- #13: Mad Dog: The bell in the Mansion has an inscription saying "Arkham Asylum April 1 1922", a reference to the day Amadeus Arkham killed Martin Hawkins.
- #14: Lucius Fox: Fox is mentioned after receiving the message to enter the Botanical Gardens and when the player upgrades the Batsuit completely.
- #15: Injustice Gang: The Injustice Gang is mentioned in a magazine with Joker's face in Harley Quinn's room in the Medical Facility; the same magazine can be seen in Arkham City in the GCPD.
- #16: Luke Oliver: Luke Oliver was the winner of a contest where the first prize winner would have their face rendered somewhere in the game, and he appeared as an inmate in the Penitentiary, his name also appearing on Joker's Party List.
- #17: Next Arkham: In the Warden's Office in the Mansion, if the player sprays all the Explosive Gel in a certain spot on the wall it opens a secret room where Quincy Sharp has been planning to build a giant prison in North Gotham called Arkham City. The blueprints also come with photos of concept art for the next game and a stamp of approval on it.
- #18: Osito: In the room before rescuing James Gordon a hint to Bane's presence appears in the form of Osito, the stuffed bear he had as a child.
- Easter Egg #19: Hush: Hush's name appears on a board in the Medical Facility showing that he works there. In an issue of Arkham Unhinged Batman actually mentions this.
- Easter Egg #20: Floronic Man: In her first interview Poison Ivy mentions Jason Woodrue. Woodrue is the one responsible for Ivy's creation and later becomes the villain Floronic Man.
- Easter Egg #21: The Three Stooges: When you are heading towards the Medical Facility, Joker randomly says "Paging Dr Howard Dr Fine Dr Howard." This is a reference to The Three Stooges when they were doctors at a hospital, using their surnames as well.
- #22: Joker's Presents: When you enter the lab in the Gardens where you will be fighting two Titan henchmen a cutscene will start. Watch it closely, since near the end look at the "presents" behind Joker. There are notes attached to them saying: "Dear Catwoman, Love Joker" and "Dear Two-Face, Love Joker". There are more notes, but they have not been "deciphered" yet. In Batman: Return To Arkham the notes all say "Dear Penguin, Love Joker", likely to justify his possession of the substance in Arkham City.
- #23: Joker's Party List: Luke Oliver, Catwoman, Two-Face, Mad Hatter, Clayface, Killer Croc, Penguin and Ventriloquist's names are all on Joker's Party List.
- #24: Arkham Mansion: The Mansion contains many references to various Batman villains throughout. Black Mask's mask can be found in Penelope Young's office, Ra's Al Ghul's body appears in the morgue and Penguin's umbrellas, Catwoman's gear, Scarface and files on Hugo Strange are located in the South Corridor.
- Easter Egg #25: Harley's Office: Inside of Harley Quinn's old office, there is a mannequin head with Harley's hat which originated in Batman: The Animated Series.
- #26: Thomas and Martha Wayne: There is a bench in the Gardens dedicated to Thomas and Martha Wayne.
- Easter Egg #27: Firefly: In the Medical Facility there is a newspaper clipping with the headline "FIREFLY ESCAPES GOTHAM ON HIGH ALERT!"
- #28: Prometheus: In the Penitentiary there is a wanted poster for Prometheus from the GCPD.
- #29: Knightfall: In one of the Game Over screens Bane is shown breaking Batman's back, referencing the infamous scene from Knightfall.
- #30: Doll's Head: In Arkham Asylum's Underside one can find a doll's head under a floor.