Alex Deuyour as himself: A 17 (later 18)-year-old wizard who is part of a prophecy where either Voldemort or Alex and Harry kill the other side and the group's brains.
Daniel Radcliffe as Harry Potter: A 17 year-old wizard who is part of a prophecy where either Voldemort or Alex and Harry kill the other side.
Rupert Grint as Ron Weasley: Alex's second cousin and godbrother who is loyal to him and Alex.
Emma Watson as Hermione Granger: One of Alex and Harry's best friends who is the group's other brains.
Kate Nguyen as herself: Alex's ex-girlfriend and a half-blood Gryffindor.
Matt Crocitto as himself: Alex's best friend and a half-blood Gryffindor.
Gary Oldman as Sirius Black: Alex and Harry's maternal uncle/honorary uncle and godfather, who checks in on them consistently.
Desiree Deuyour as herself: Sirius' younger sister and Alex's mother, an Auror and member of the Order of the Phoenix.
Helena Bonham Carter as Bellatrix Lestrange: Alex and Sirius' insane cousin and a Death Eater in Voldemort's inner circle.
Robbie Coltrane as Rubeus Hagrid: Alex and Harry's half-giant friend, member of the Order, and Gamekeeper/Care of Magical Creatures teacher at Hogwarts.
Warwick Davis as Griphook: A goblin and former employee at Gringotts Bank.
Ralph Fiennes as Lord Voldemort: An evil power-hungry dark wizard and the leader of the Death Eaters.
Michael Gambon as Albus Dumbledore: The deceased Headmaster of Hogwarts, who Snape killed in the previous film.
Brendan Gleeson as Alastor "Mad-Eye" Moody: A grizzled paranoid Auror and member of the Order of the Phoenix.
Richard Griffiths as Vernon Dursley: Harry's magic-hating Muggle uncle.
John Hurt as Garrick Ollivander: A wandmaker abducted by the Death Eaters.
Jason Isaacs as Lucius Malfoy: Draco's father and a disgraced Death Eater.
Alan Rickman as Severus Snape: Alex's godfather and honorary uncle, who used to favor him, a double agent supposedly working for Voldemort, and the newly appointed Headmaster of Hogwarts.
Fiona Shaw as Petunia Dursley: Harry's Muggle aunt.
Timothy Spall as Peter Pettigrew / Wormtail: The Death Eater who betrayed Harry's parents to Voldemort.
Imelda Staunton as Dolores Umbridge: Senior Undersecretary to the Minister of Magic and the appointed Head of the Muggle-Born Registration Commission.
David Thewlis as Remus Lupin: A werewolf and member of the Order of the Phoenix.
Julie Walters as Molly Weasley: A mother-figure to Harry and Ron's mother.
The film also features Bonnie Wright as Ginny Weasley, Ron's sister and Harry's love interest; Matthew Lewis as Neville Longbottom, one of Alex and Harry's friends; Evanna Lynch as Luna Lovegood, another one of Alex and Harry's friends and Kate and Ginny's best friend; Tom Felton as Draco Malfoy, Alex's first cousin and Harry's Death Eater school rival, and Helen McCrory as Narcissa Malfoy, Draco's mother and Bellatrix's sister. Mark Williams, James and Oliver Phelps, Domhnall Gleeson and Clémence Poésy appear as members of the Weasley family and the Order of the Phoenix Arthur Weasley, twins Fred and George Weasley, Bill Weasley and Fleur Delacour, respectively. Natalia Tena, George Harris and Andy Linden appear as members of the Order of the Phoenix Nymphadora Tonks, Kingsley Shacklebolt and Mundungus Fletcher, respectively.
Other cast members include Bill Nighy as Minister of Magic Rufus Scrimgeour; Rhys Ifans as Luna's eccentric father Xenophilius Lovegood; Hazel Douglas as Voldemort's familiar snake Nagini, who puppeteers the corpse of Bathilda Bagshot; David Ryall as Albus Dumbledore's old school friend Elphias Doge; Frances de la Tour as head of Beauxbatons Madame Maxime; Guy Henry as Pius Thicknesse, a puppet Minister of Magic installed by Voldemort; Peter Mullan as Corban Yaxley, a Death Eater at the Ministry of Magic; and Carolyn Pickles as Charity Burbage, the Muggle Studies teacher at Hogwarts, tortured and killed by Voldemort.