 Watson, Radcliffe and Grint last appeared together in 2005 |
Radcliffe, 17, Rupert Grint, 18, and Emma Watson, 16, made their debuts as young wizards in Harry Potter and the Philosopher's Stone in 2001.
Warner Brothers chief Jeff Robinov said it would be "inconceivable" to have anybody else in the roles.
Radcliffe's role was confirmed by the studio earlier this month, but it had been rumoured Watson would not return.
The trio have appeared in four movies, and will star in the fifth Potter film, The Order of the Phoenix, in July.
The final two films, to be based on JK Rowling's books, will follow suit - The Half-Blood Prince, scheduled for release in November 2008, and The Deathly Hallows. 
There had been some speculation that the three actors were becoming too old to fulfil their parts. Radcliffe and Grint, who plays Ron Weasley, have moved onto adult roles on stage and screen, while it had been rumoured that Watson had told producers she did not want to continue.
But she said: "I could never let Hermione go - she is my hero! I love her too much and love what playing her has meant to me. I am excited and honoured to be finishing what I started and playing her in all seven of the films."
Radcliffe, who is starring in Equus on the London stage - a part that requires him to appear nude - said playing Potter was an "enormous privilege".
"I feel a huge sense of loyalty to the character of Harry and the fans who have supported these films over the years," he added.
Mr Robinov, the studio's president of production, said: "Through the years we have watched them grow into extraordinary young adults, as well as remarkable actors. 
Kumkum Bhagya
17