Alex Edgbert as himself: A 13 (nearly 14) year-old British-American wizard and the brains of the group who believes in Sirius' innocence.
Daniel Radcliffe as Harry Potter: A 13-year-old British wizard famous for surviving his parents' murder at the hands of the evil dark wizard Lord Voldemort as an infant, who now enters his third year at Hogwarts School of Witchcraft and Wizardry.
Rupert Grint as Ron Weasley: Alex's godbrother and first cousin and one of Alex and Harry's best friends at Hogwarts and a younger member of the Weasley family.
Emma Watson as Hermione Granger: Alex and Harry's other best friend at Hogwarts and the other brains of the group.
Kate Nguyen as herself: Alex's crush and best friend and a half-blood Gryffindor.
Matt Crocitto as himself: Alex's best friend and a half-blood Gryffindor.
Robbie Caltrone as Rubeus Hagrid: The gamekeeper and new Care of Magical Creatures teacher at Hogwarts.
Michael Gambon as Albus Dumbledore: The headmaster of Hogwarts and one of the greatest wizards of all time.
Richard Griffiths as Vernon Dursley: Harry's Muggle uncle.
Gary Oldman as Sirius Black: Alex's godfather and maternal uncle and Harry's infamous godfather, who escapes from Azkaban after serving twelve years there for being wrongly accused of being the Death Eater who betrayed Harry's parents to Voldemort.
Alan Rickman as Severus Snape: Alex's godfather and honorary uncle, the Head of Slytherin House and Potions professor who favors Alex.
Fiona Shaw as Petunia Dursley: Harry's Muggle aunt.
Maggie Smith as Minerva McGonagall: Deputy Headmistress of Hogwarts, the Transfiguration teacher at Hogwarts and head of Gryffindor.
Timothy Spall as Peter Pettigrew: A former friend of Harry's parents said to have been killed by Sirius Black, but was later revealed to have been the real Death Eater who betrayed Harry's parents to Voldemort.
David Thewlis as Remus Lupin: The new Defense Against the Dark Arts teacher at Hogwarts and a werewolf.
Emma Thompson as Sybil Trelawney: The Divination teacher at Hogwarts.
Julie Walters as Molly Weasley: Ron's mother.
Desiree Deuyour as herself: An Auror and Alex's mother, who believes Sirius is innocent.
Several actors from the previous film reprise their roles in Prisoner of Azkaban. Harry Melling appears as Dudley Dursley, Harry's cousin. James and Oliver Phelps play Fred and George Weasley, Ron's twin brothers; Chris Rankin appears as Percy Weasley, Ron's other brother and a Hogwarts head boy; and Bonnie Wright portrays their sister Ginny, while Mark Williams plays their father, Arthur Weasley. Tom Felton portrays Draco Malfoy, Harry's rival in Slytherin, while Jamie Waylett and Josh Herdman appear as Crabbe and Goyle, Draco's minions. Matthew Lewis and Devon Murray play Neville Longbottom and Seamus Finnigan respectively, two Gryffindor students in Harry's year. David Bradley appears as Argus Filch, Hogwarts' caretaker, while Robert Hardy portrays Cornelius Fudge, the Minister for Magic.
Pam Ferris portrays Aunt Marge, Vernon's sister; Lee Ingleby plays Stan Shunpike, conductor of the Knight Bus; and Jim Tavaré appears as Tom, innkeeper of The Leaky Cauldron, replacing Derek Deadman from the first film. Dawn French plays the Fat Lady, a painting at Hogwarts, replacing Elizabeth Spriggs from the first film. Julie Christie appears as Madam Rosmerta, the barmaid at the Three Broomsticks. Warwick Davis appears as the conductor of the Hogwarts choir.