- "Technically he's not a serial killer, yet. Serial killing involves the murder of several people in three or more separate events. So far our guy has only done two..."
- —Dexter Morgan regarding the Doomsday Killer.
A Serial Killer is a person who murders three or more people with a "cooling off" period between each murder and whose motivation for killing is largely based on psychological gratification. This sets them apart from a Murderer who simply kills one or two victims, for professional reasons (including law enforcement, not just hired killers) or kills someone in cold blood for personal purposes. Lila West is not a serial killer as she only caused two deaths as a part of her pyromaniac tendencies and both were personal (in a sense).
As part of The Code of Harry, Dexter Morgan will target and kill murderers. Rather than use the strict definition of "three or more" victims, Dexter will sometimes kill someone who has killed only once and is likely to kill again. 
Each episode of Dexter usually revolves around Dexter either attempting to kill or trailing a serial killer or Murderer. A common trait in the television series is that a serial killer is usually what is referred to as the Main Antagonist for each season or at least this is what is commonly interpreted. Season Two does not present a serial killer as a main antagonist but rather multiple antagonists dealing around Dexter's activities becoming known to the public (with the killer being considered the Bay Harbor Butcher). Season Three presents George King as the serial killer antagonist but more in tune with Debra Morgan and the rest of Miami Metro Homicide. Miguel Prado, who is simply a Murderer, is the main antagonist. Season Five depicts multiple serial rapists however it's unclear if any of them actually killed the girls except for Boyd Fowler, who psychologically believed he was ending their suffering. Season Six depicts a religious serial killer, Travis Marshall (who is known to public as the Doomsday Killer), his killings depict the tableau's from the Book of Revelation with Travis being one of the witnesses (another being his dark passenger, Professor James Gellar) trying to bring about the end of the world, although his efforts became useless as he was killed by Dexter Morgan in the end. Season Seven features a Ukrainian Crime Syndicate known as The Koshka Brotherhood as the Main Antagonists (instead of a serial killer). Season Eight features a new serial killer known as The Brain Surgeon whose identity is revealed to be Oliver Saxon.
Known Serial Killers
- Dexter Morgan / Bay Harbor Butcher - Dexter himself, a serial killer named in Season 2, but active in all seasons, notorious for murdering other murderers and serial killers and disposing of their dismembered corpses at the bottom of the bay. Thought to be inactive.
- Mike Donovan - Serial killer in Season 1 who raped and murdered young boys.
- Brian Moser / Rudy Cooper / The Ice Truck Killer - Dexter's brother, a serial killer in Season 1 known for his modus operandi of dismembering prostitutes, thought to have conducted his murders in an ice truck, hence his name.
- Mary - Serial killer seen in a flashback in Season 1. Nurse who overdosed her patients with morphine as a form of euthanasia.
- Jorge and Valerie Castillo - Serial killer couple in Season 1 who transported Cuban immigrants across the border, killing those who didn't pay.
- Alex Timmons - Serial killer in a flashback in Season 1. Former sniper in the Marines who killed innocent children.
- Cindy Landon - Serial killer in a flashback in Season 1. "Black widow" who married and killed her elderly, wealthy husbands for their money.
- Emmet Meridian - Serial killer therapist in Season 1 who indirectly killed several wealthy, powerful women by coercing them to commit suicide.
- Jimmy Sensio - Serial killer and blind voodoo high priest who was hired to poison people in Season 2.
- Alfonso Concepcion / Little Chino - Serial killer and leader/enforcer of the Twenty-Ninth Street Kings in Season 2.
- George Washington King / Jorge Orozco / Skinner - Serial killer in Season 3 in who tortured his victims by slowly cutting off their flesh before killing them.
- Billy Fleeter - Serial killer in Season 3 who killed to pay off his gambling debts.
- Arthur Mitchell / Trinity - Serial killer in Season 4 who conducted a cycle of specific kills based on his own tragic childhood.
- Johnny Rose and Nikki Wald / Vacation Murderers - Serial killers who robbed and later murdered tourists in Season 4.
- Timothy Brand - Serial killer who murdered women in Season 4.
- Boyd Fowler - The serial killer of the rapist team, the Barrel Girl Gang in Season 5.
- Carlos and Marco Fuentes / Fuentes Brothers / Santa Muerta Killers - Serial killers who robbed and brutally murdered couples in Season 5.
- Lance Robinson - Serial killer in Season 5 who contacted homosexual men on the internet for sexual activities and killed them.
- Travis Marshall / Doomsday Killer - Religious serial killer with a split personality who conducted kills based on the Doomsday Tableaus in Season 6.
- Julio Benes - Serial killer in Season 6 and leader of the Locos who killed those who opposed his gang.
- Walter Kenney / The Tooth Fairy- Serial killer with the modus operandi of killing prostitutes and removing their lateral incisors in a string of killings in Portland, Oregon. Resurfaces in Miami years later in Season 6.
- Ray Speltzer - Serial killer of single women in a ritual based on the minotaur mythos in Season 7.
- Wayne Randall - Serial killer known for a killing spree from fifteen years ago, appearing in Season 7.
- Hannah McKay - Serial killer in Season 7, who was an accomplice of Wayne Randall who continued killing after her partner was incarcerated.
- Oleg Mickic - Serial killer hitman for the Koshka Brotherhood in Season 7, who was an ex-soldier and sniper.
- Benjamin Caffrey - Serial killer hitman for the Koshka Brotherhood in Season 7, who slit his vicitms' throats with knives and wire.
- Joe Jensen / The Phantom Killer - Serial killer in Season 7 who killed people by burning them alive.
- Oliver Saxon/The Brain Surgeon - Serial killer in Season 8 who carves out a portion of his victims' brains, most often after he has already killed them though occassionally while they are still alive.
- A.J. Yates - Serial killer who kidnaps women and breaks all of their toes before killing them in Season 8.
- Dexter Morgan
- Father Donovan
- Tamiami Slasher - The name given to Dexter's brother in the book Darkly Dreaming Dexter.
- Brandon Weiss - The serial killer in Dexter by Design.
- Bobby Acosta - One of the cannibalistic serial killers in Dexter is Delicious.
- Alana Acosta - The leader of the cannibals in Dexter is Delicious.
- A common trait among serial killers seems to be getting up early in the morning. Dexter comments on this in Episode 109: Father Knows Best, shortly before running into Brian, who had gotten up before him. Later in the series, in Episode 408: Road Kill, he meets with Arthur Mitchell, who comments that he is an "early riser".