Gushie (Dennis Wolfberg) - Project Quantum Leap's head computer programmer and senior operations coordinator. A quirky man, Gushie is often described as "a little guy with bad breath". He is the butt of jokes between Sam and (especially) Al. When he is seen, he tends to be in a state of worry due to Sam's present situation. Gushie once filled in for Al as Sam's holographic contact when Al was busy attempting to track down a murderer who escaped the waiting room.
Ziggy (voiced by Deborah Pratt) - The supercomputer Sam designed to run Project Quantum Leap. The computer has a feminine personality and a massive ego; the latter feature is an accomplishment Sam is particularly proud of, as it makes Ziggy much more than a simple number-crunching machine. In early episodes, it is suggested that Ziggy is the computer's operator. But when Sam speaks to Ziggy directly, it shows that Ziggy is a self-operating computer who is capable of recognizing and expressing human emotions. Ziggy is programmed with historical information through Sam's life and has access to outside databases, and is able to project the likely effect of Sam's actions on history. While Al is with Sam, he gets information from Ziggy through a faulty handheld device. Al can get information from Ziggy through this device, or by simply asking out loud (Al sees whatever location Sam is in as a holographic projection and is always physically in close proximity to Ziggy.
Dr. Donna Eleese (Teri Hatcher, age 19 / Mimi Kuzyk, age 45) - Born Donna Wojohowitz, she is Sam's wife and director of Project Quantum Leap in her husband's absence. Originally, Donna was not involved with the project, until Sam altered history, thus ensuring her involvement and getting rid of the Committee (see below).
Dr. Verbena Beeks (Candy Brown Houston) - The project's head psychiatrist. It is her job to work directly with the person Sam displaces when he leaps as they are often in a state of shock and panic when they arrive.
Tina Martinez (Gigi Rice) - Tina is a medical technician that works on the Project. No one can enter the Imaging Chamber or the Accelerator Chamber without first being inspected by her. She is Al's girlfriend and is also having a secret affair with Gushie. In the episode where Sam accidentally erased Al from existence, she was married to Gushie.
Edward St. John V (Roddy McDowall) - When Sam changed history and accidentally erased Al from existence, Edward St. John V appeared as his holographic contact. After correcting the error, Al reappeared. Whether or not St. John works on the Project outside of that alternate timeline is unknown.
Samantha Josephine "Sammy Jo" Fuller (Kimberly Cullum, age 11) - Project technician who works on the project. She is the biological daughter of Sam and Abigail Fuller, conceived during a leap into Abigail's fiancé in 1966. Only the child-version of Sammy Jo is ever seen. Sammy Jo spends her life believing that Abigail's former fiancee is her biological father, and she is never told anything different. Sam figures out that Sammy Jo is his daughter, and Al confirms this through Ziggy. Child Sammy Jo resembles Abigail at the same age, but with Sam's hair colour. She has a fascination with time travel and has a photographic memory, and bonds easily with Sam. Sam has no memory of meeting Adult Sammy Jo, even though she works on the project with him. Ziggy's prediction that Sam will forget this secret when he has his next leap comes true. This means that only Al knows the truth about Sammy Jo's parentage.
Senator (Warren Frost) - The head of a government oversight committee that tried to shut down Project Quantum Leap when Sam failed to stop the downing of a U-2 spy plane.
Senator Diane McBride (Alice Adair) - Although he failed to stop the U-2, Sam did change history and Senator Diane McBride became the head of the oversight committee and ruled to continue the Project's funding. Al witnesses the former Senator morph into Diane. He immediately realizes that his was the result of Sam changing the timeline, but is the only person who is aware of it. He never mentions what he witnessed to anyone.
The Committee - Led by men named Weitzman and Bartlett, the committee oversaw Project Quantum Leap until Sam altered history, bringing Donna onto the Project, assuring government funding and eliminating the need for the Committee.
John Samuel Beckett (Scott Bakula) - Sam's father, a gruff but kind dairy farmer in Elk Ridge, Indiana. He was named after his grandfather, a Civil War officer. When Sam leaps into his teenage self, he recalls that his father had died a few years later due to poor health. Sam fails in getting him to quit smoking. However, Sam would encounter his much slimmer father in another leap many years after his father's death. This confirms that Sam managed to convince his father to make better dietary choices, thus improving his health and greatly extending his life.
Thelma Louise Beckett (Caroline Kava) - Sam's mother. After her husband's death, she moved to Hawaii to live with her daughter and her second husband.
Lieutenant Thomas "Tom" Beckett (David Newsom) - Sam's older brother who was killed in Vietnam on April 8, 1970 until Sam changed history and kept his brother alive. However, this resulted in Al not being rescued and being held as a prisoner of war. His first wife then believed Al to have been killed in action and moved on with another husband, not knowing that Al was trying to get back to her.
Katherine "Katie" Beckett (Olivia Burnette) - Sam's younger sister. To get away from trouble at home, she married an abusive alcoholic at a young age. Years later she divorced him and eventually married an Air Force officer, Lieutenant Jim Bonick. After John Beckett's death, Thelma lived with Katie and Jim.
Beth Calavicci (Susan Diol) - Al's first wife and the only woman he ever truly loved. When Sam meets Beth during a Season 2 leap, she believes that she is widowed as her husband, who was serving in military action in the Vietnam war, had been reported missing-in-action and presumed dead. Al pressures Sam into stopping her forming a relationship with another man, insisting that "her husband" was still alive. Sam figures out Beth's identity and refuses to fulfill Al's request, deeming it to be selfish. In his next two leaps, Sam tries to save his own brother in the Vietnam war. This accidentally thwarted a rescue attempt that would've rescued Al, causing him to become a war prisoner. When Al is freed and returns home, he learned that Beth thought he was dead and had remarried. So he chose to leave her alone. Sam is given a chance to put this right, and then assures Beth that her husband Al was alive and is trying to get back to her.
Sammy Jo Fuller - Sam's daughter due to his leaping. See 'Project Quantum Leap' above.
Captain John Beckett (Backula) - Captain John Beckett is a civil war officer and direct ancestor of Sam. Sam leaps into him, making him the only leapee to have existed outside of Sam's own time. While he represents the North, he is taken hostage by supporter of the Southern Army. Al confirms through Ziggy that his abductor is also his ancestor, and must ensure that she falls in love with the Captain. Otherwise Sam would prevent his own birth and potentially stop Project Quantum Leap from ever beginning and undoing every change Sam ever made.
In Season 5, Sam discovered a leaper from another project. This leaper project seemed to follow the same structure and scientific principles as Quantum Leap, but was dedicated to the opposite goal of putting wrong what once went right. They sometimes make reference to unspecified authority figures who will punish them greatly if they fail.
Alia (Renée Coleman) - Sam's counterpart, a beautiful and deadly woman who is leaping around in time, committing terrible acts in an attempt to get home. Alia and Sam see each other as their respective leapees until they make physical contact. The first time Sam touches Alia, their genuine identities are revealed simultaneously. On the second occasion when they shake hands, Alia is able to see Sam but Sam still sees her as her leapee until he makes contact again. After meeting and failing to kill Sam twice, Alia abandons her old ways and tries to escape the ones controlling her leaping with Sam's help. At the end of "Revenge of the Evil Leaper" her leap is accompanied by a blue flash rather than her usual red flash, implying she has escaped the Evil Leaper project. It is implied that Alia was an unwilling participant in the project.
Zoey (Carolyn Seymour) - Alia's holographic partner. She is a cruel and sadistic woman who, after Alia breaks free of their project's control on her, leaps back in an attempt to kill Alia herself. Unlike Sam and Al, Zoey is clearly in charge of her respective pairings. She manipulates Alia and cares little about her feelings. While Zoey is a hologram, she can see Sam if Alia can (due to her being tuned in to Alia's brainwaves), but Sam remains unable to see or hear her.
Thames (Hinton Battle) - When Zoey leaped, a darkly comical man named Thames became her hologram.
Lothos - The other project's computer system, analogous to Ziggy.