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". 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. 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.
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 Al knows the truth about Sammy Jo's parentage.
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.
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.
Sen. Diane McBride (Alice Adair) - Although he failed to stop the U-2, Sam did change history and Sen. Diane McBride became the head of the oversight committee and ruled to continue the Project's funding.
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.
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.
Lt. 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.
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, Lt. 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.
Sammy Jo Fuller - Sam's daughter due to his leaping. She is a child prodigy who grows up to work at Project Quantum Leap however she does not know that Sam is her father.
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.
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. 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.
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.