They join stars such as John Lithgow, who will play Dumbledore, Nick Frost as Hagrid, Janet McTeer as Minerva McGonagall and Paapa Essiedu as Severus Snape.
The three child actors who will play Harry, Ron and Hermione were announced in May.
The programme will be filmed at Warner Bros Studios Leavesden in Hertfordshire, which is where the eight Harry Potter movies were also shot.
HBO has previously said the series would be “a faithful adaptation of the iconic books”.
The TV series will have more breathing space to explore the plot lines from the books without the time constraints of the film.
Producers also announced new production staff, several of whom have previously worked on The Crown such as director of photography Adriano Goldman and hair and makeup designer Cate Hall.
The first series will launch in 2027.