- Qyburn: A former maester of the Citadel who was stripped of his chains for practicing unethical and illegal human experimentation. Later, he serves Queen Cersei Lannister that allows him to conduct all kinds of bizarre researches, some of them at her request. He was able to restore Ser Gregor Clegane that was in the brink of death, transforming him into an undead man to serve and protect Cersei.
- Heinz Doofenshmirtz: Heinz Doofenshmirtz is self-proclaimed mad scientist whom schemed his evil plans (which ranged from petty ones into more or less cataclysmic ones) by building various machines called Inators. While many of the scheme inspired by various mistreatment that he endured in his youth, Doofenshmirtz himself is not a heartless mad scientist and found his intended crime would be amusing had his nemesis Perry the Platypus a.k.a. Agent P comes to stop him even if the intended crime proved highly trivial.
- Professor Baxter Stockman: He started off from creating Mousers, cat-sized chicken/theropod dinosaur-like robots that he claimed to help rat problems in New York, but later revealed to had them performing massive-scale burglary for him.
- Dr. Mindbender: He is a member of Cobra organization and a mad scientist who created the B.A.T.S. and Serpentor the Cobra Emperor by using the remains of noble dead conquers of the world.
- Dr. Drakken: He is a blue-skinned mad scientist and super villain obsessed with world domination, as a result of being picked on by his colleagues in college. Despite being clueless, Drakken is capable of building sentient robots, super-lasers, beam cannons, and other such "evil" technologies.
- Dr. Boycott: He is the chief scientist of A.R.S.E. (Animal Research: Scientific and Experimental). He tortures and mistreats animals there, including Rowf and Snitter and has no remorse for the animals or his subordinate, Stephen Powell. He has done many endurance experiments against Rowf by drowning him in a tank and has done many vivisection experiments on Snitter by plaguing his conscious and subconscious mind together, causing him to have hallucinations.
- Professor Preterius: A scientist who tries to create a formula that would heal wounds. Preterius eventually went mad and bankrupt due to his repeated failures, talking to Claudandus like an old friend, and raving about a new, "perfect" breed of cats. Claudandus killed Preterius after the mad doctor "heard" him demanding to be released for a battle and complied.
- Shou Tucker: An alchemist that used his wife and later his own daughter to create chimeras with the ability to speak, so he could secure his State Alchemist’s License.
- Dr. Merlot: The founder and head of Merlot industries who believes that Grimm can be controlled to benefit humanity and mutates a handful of Grimm making them more dangerous.
- Caesar Clown: A wanted criminal and a mad scientist. While working for Doflamingo, he created artificial Devil Fruits to empower Kaido’s army. Upon Big Mom’s request, he kidnapped children and conducted gigantification experiments on them. His specialty is creating chemical weapons of mass destruction.
- Orochimaru: A ninja that desired to learn all the world's secrets, so he sought to become immortal to live longer and accomplish this goal. He conducted illegal experiments with corpses, and used people that he kidnapped (including children and newborns) as test subjects. From his research he found a way to cheat death by transferring his own soul into somebody else's body.
- Amshel Goldsmith: A scientist that conducted unethical experiments using both humans and chiropteran vampires. He was Joel I’s assistant on experimenting chiropterans, and witnessed the birth of the two chiropteran queens (Saya and Diva). He and Joel locked Diva in a tower and performed terrible experiments on her. They also attempted to induce Diva and Saya to mate with humans. Later, Amshel became the first of Diva’s Chevaliers after drinking her blood and he took over the chiropteran research after Joel’s death. Using the blood of his main research subject (Diva), he worked with a pharmaceutical company to turn all the humans of the world into chiropterans.
- Van Argiano: The head scientist of a pharmaceutical company owned by Amshel and the Goldsmiths. When products of this company are consumed by humans, they turn into chiropteran vampires after hearing Diva's song.
- Baron K. Roolenstein: He is the evil King K. Rool's scientist alter-ego when he create the machine KAOS to seek revenge against the Kong Family and restore the Kremling Krew to their former glory.
- Terra-Xehanort: He is Master Xehanort possessing Terra’s body and worked as one of the apprentices of the scientist Ansem the Wise. Without his mentor knowing, he manipulated the other apprentices to conduct inhumane experiments on the human heart and the darkness within it. Eventually, all the apprentices were consumed by darkness and split into Heartless and Nobodies.
- Vexen/Even: He is a Nobody of the Organization XIII, but previously he was once one of Ansem the Wise’s apprentices as his complete self (Even). As his original self, Even conducted inhumane experiments on the human heart and the darkness within it until his own heart was consumed by darkness. As a Nobody (Vexen) working for the Organization, he was responsible for the Replica Program and the creator of several replicas, including Riku Replica and Xion.