The move was made to take advantage of an open weekend in Hollywood's busy summer season, said Alan Horn, Warner Bros. president and chief operating officer. The film had been on schedule, and the change was not due to any production snags, he said.
"The picture is completely, absolutely, 100 percent on schedule, on time. There were no delays," Horn told The Associated Press. "I've seen the movie. It is fabulous. We would have been perfectly able to have it out in November."
The switch will mean a two-year lag between the film adaptations of books five and six in J.K. Rowling's fantasy series. But it will shorten fans' wait between "Half-Blood Prince" and the final two installments, which are being shot simultaneously next year.
Based on book seven, "Harry Potter and the Deathly Hallows," the last two movies are due out in close order, in November 2010 and summer 2011. Horn said the later release of "Half-Blood Prince" will not affect the schedule for the final two movies.
Horn said the studio has had success with past summer "Harry Potter" releases, including the fifth movie, which was released in 2007 and became the second-highest grossing in the franchise.
The recent Writers Guild of America strike also had affected Hollywood's lineup in next summer, leaving a key date open for Warner to slot in "Half-Blood Prince," Horn said.
The July 17 release will be over the same weekend that Warner debuted this year's blockbuster "The Dark Knight," which had a record-breaking opening weekend and is on its way to $500 million domestically and the No. 2 spot on the all-time box-office charts behind "Titanic."
"Half-Blood Prince" finds Harry (Daniel Radcliffe) returning to his wizard classes with a clandestine assignment to root out dark secrets about the early years of his archenemy, the dark Lord Voldemort.
The film is directed by David Yates, who made the fifth movie and also is shooting the final two.
Last February, another big Hollywood film, Paramount's "Star Trek," was bumped from a December release to May 8, 2009. Paramount executives said that move also was intended to take advantage of an open weekend in the summer lineup.
