Recurring
Carlos Compean as Arturo Ortecho, Liz's father, who runs the Crashdown Cafe diner. He is an undocumented immigrant from Mexico and has been hiding his undocumented status for nearly 30 years.
Peter Diseth as Grant / Graham Green, an alien theorist and podcaster and his twin brother, a proprietor of an alien-themed museum
Kayla Ewell as Nora Truman, Michael's biological mother who was present at the UFO crash in 1947
Rosa Arredondo as Michelle Valenti (seasons 1–3), Kyle's mother, who succeeded her late husband, Jim Valenti, as sheriff of Roswell
Dylan McTee as Wyatt Long (seasons 1–3), the brother of one of the girls Rosa Ortecho killed in a car accident, who harbors a grudge against her family
Riley Voelkel as Jenna Cameron (seasons 1–2, 4), Max's fellow deputy and partner In the fourth season, she joins the FBI.
Sherri Saum as Mimi DeLuca (seasons 1–2), Maria's mother, who suffers from an undiagnosed mental illness typified by some temporary memory loss and her belief in aliens
Claudia Black as Ann Evans (seasons 1–2), Max and Isobel's adoptive mother
Kiowa Gordon as Flint Manes (seasons 1–2), Alex Manes' older brother
Justina Adorno as Steph (season 2), the daughter of the hospital's director of Surgery who catches Kyle's attention
Jason Behr as Tripp Manes (season 2), Alex Manes' great-uncle who was present at the UFO crash in 1947
Cassandra Jean Amell as Louise Truman (seasons 2–3; guest season 4), Isobel's biological mother who was present at the UFO crash in 1947
Gaius Charles as Roy Bronson (seasons 2–3; guest season 4), a farmer from 1947 who befriends Nora and falls in love with Louise
Jamie Clayton as Grace Powell / Charlie Cameron (season 2), Jenna Cameron's sister who is on the run from the military
Cleo Anthony as Diego (season 2), Liz's ex-fiancé whom she left in Colorado prior to her return to Roswell
Christian Antidormi as Forrest Long (seasons 2–3), Wyatt Long's cousin and a love interest of Alex Manes
Tanner Novlan as Gregory Manes (seasons 2–3), Alex Manes' protective older brother
La'Charles Trask as Bert (seasons 2–3)
Steven Krueger as Heath (season 3, guest season 4), Liz's colleague at Genoryx
Sibongile Mlambo as Anatsa (season 3–4), an investigative journalist
David DeSantos as Edgar (season 3)
Michael Grant Terry as Jordan Bernhardt (season 3), the son of Roswell's mayor
Gillian Vigman as Brooke Taylor (season 3), the town sheriff
Quentin Plair as Dallas (season 3–4)
Andrew Lees as Clyde (season 4)
Zoe Cipres as Bonnie (season 4)
Rekha Sharma as Dr. Shivani Sen (season 4)
