Dial M for Monkey is a segment of Dexter's Laboratory starring Dexter's pet lab monkey, who has superpowers and fights crime with his lovely partner Agent Honeydew and the commander General. Dexter wants his pet lab monkey to have some sort of strange thing about his but he never finds anything, but whenever Dexter isn't looking, Monkey turns into a super powered monkey named Monkey who fights crime and evil whenever all of the other forces have been beaten. Although Monkey is a part of his own segment, he has been seen in the Dexter's Lab segment multiple times.
- Monkey - Monkey is the main protagonist of the series. He is a normal old lab monkey who Dexter has in his lab but he has a superhero secret identity. Whenever nobody is around, Monkey transforms into a superhero named Monkey who is super amazing and cool, while he is pretending to be normal, he acts stupid and seems to like doing stupid girly things with Dexter's sister Dee Dee. He is voiced by Frank Welker.
- Agent Honeydew - Agent Honeydew is Monkey's sidekick. She is a S.H.I.E.L.D. agent who is good at action and using weaponry. She speaks with an English accent.
- Commander - He is the leader of the S.H.I.E.L.D. agents who always appears on a monitor. He alerts his agents of impending doom and sends them out to stop them. He loves Monkey the most.
- Quackor - Quackor is a duck who is Monkey's girlfriend. She was once a villain of his, but now she has revealed herself to be female and they fell in love. She is voiced by Frank Welker.
- The Justice Friends - The Justice Friends are main characters in their own segment of the show, but they are recurring characters in this segment also. The Justice Friends are a team of superheroes that Monkey is a part of.
- Magmanamus (voiced by Brad Garrett) - The eponymous antagonist of the episode of the same name, he is a cranky lava monster who wants to get some sleep, but can't due to the noisy citizens.
- Simion (voiced by Maurice LaMarche) - Appearing only in Dial M for Monkey: Simion, he is a highly intelligent chimpanzee who had it rough being a lab monkey and vowed revenge on the human race for how he was treated. He soon changed his ways when Monkey gave him a banana, which reminded him of his joyful past.
- Rasslor (voiced by the late Randy "Macho Man" Savage) - An alien being who strived to be the greatest wrestler in the universe. He defeated most of the heroes of Earth until his fight with Monkey. Due to Monkey not wanting to stay down, Rasslor decides to spare his planet. He is a parody of The Champion of the Universe who first appeared in Marvel Two-In-One Annual #7.
- Promotor (voiced by Frank Welker) - An alien fly who was Rasslor's manager. He is a parody of Don King.
- Barbequor (voiced by Robert Ridgely) - An alien being who is the devourer of worlds. Barbequor plotted to devour the entire Solar System and repelled anyone who tried to disrupt his barbecue through whatever way possible. Monkey managed to get him to spit it out. He is a parody of Galactus.
- Silver Spooner (voiced by Rob Paulsen) - A silver alien that rides on a giant spoon and is a Herald of Barbequor. He is a parody of the Silver Surfer.
- Orgon Grindor (voiced by Jim Cummings) - An alien organ grinder who used his barrel organ to manipulate Monkey into stealing the gold from Fort Knox.
- Huntor (voiced by Ed Gilbert) - An alien lion-like hunter who wants to kill Monkey for use as a hunting trophy. Huntor later made a cameo in an episode of Samurai Jack as one of the bounty hunters that try to capture the title character.
- Peltra (voiced by Dee Dee Rescher) - An alien woman obsessed with making fur coats from endangered species across the universe.
- Skinner (voiced by Tom Kenny) - One of Peltra's henchmen, the skinnier of the two, known for skinning creatures and possessing dangerous weapons.
- Tanner (Frank Welker) - One of Peltra's henchmen, the more muscular of the two.
Monkey's true identity is revealed to Dexter in the episode Last But Not Beast although Dexter's memories are erased shortly afterward.
- Due to the show's superhero-like nature, it is a spoof of DC Comics' book Dial H for Hero. The name is also a reference to the 1954 Alfred Hitchcock film Dial M for Murder.
- Most of the villains in this show have names that end in "Or".