Moviegoers 18 and older accounted for 43 percent of the audience, while parents with children made up 27 percent, according to distributor Warner Bros.
Overture Films earned the No. 2 spot with Jamie Foxx and Gerard Butler's vengeance thriller "Law Abiding Citizen," which debuted with $21.3 million.
Expanding into wider release, Paramount's low-budget horror sensation "Paranormal Activity" moved up to No. 3 with $20.2 million.
Shot for a reported $15,000, "Paranormal Activity" outdid the premiere of Sony's fright flick "The Stepfather," which cost $19 million and played in nearly four times as many theaters but managed just a No. 5 opening with $12.3 million.
The results for "Where the Wild Things Are" matched the intent of director Spike Jonze, who viewed his take as a story about a child, but not necessarily a children's movie.
During production, Jonze had clashed with Warner Bros., which had wanted a more kid-friendly film. The studio gave Jonze more time and money to finish the film and ultimately backed his vision with a huge marketing campaign for "Wild Things."
"I think all sides reached a very happy compromise, and certainly Spike delivered a movie that was so true to the book, yet it generated the emotion that we felt strongly about to bring in our family audience, as well," said Dan Fellman, head of distribution for Warner.
Jonze's adaptation features newcomer Max Records as Sendak's misbehaving young protagonist, a boy who journeys to a make-believe island of monsters torn between hugging him and eating him. The live-action and voice cast includes Catherine Keener, James Gandolfini and Forest Whitaker.
A cheap acquisition at the Slamdance Film Festival, "Paranormal Activity" came out of nowhere, riding online fan buzz to a domestic total of $33.7 million so far. The movie expanded to 760 theaters, up 600 from the previous weekend, and has plenty of room to grow.
Paramount plans to expand the movie to between 1,800 and 2,000 theaters next weekend, then widen its release even farther for Halloween. It will go head-to-head with an established horror franchise as Lionsgate opens "Saw VI" on Friday.
Shot in a raw documentary style, "Paranormal Activity" is a twist on the haunted house story as a couple tries to capture on camera the strange phenomena and apparitions afflicting them.
"Paranormal Activity" might have a shot to duplicate the success of "The Blair Witch Project," a 1999 Sundance Film Festival discovery that rode Internet buzz to a $140 million domestic total.
"When you have a movie playing this well and it has such a broad appeal, it certainly tells you that is a possible outcome," said Rob Moore, vice chairman of Paramount. "I certainly couldn't predict it yet, but nothing with this movie has been predictable so far."
Hollywood had its strongest weekend yet this fall, with overall business at $141 million, up 41 percent from the same weekend last year.
"All the top five movies all did really well. It's kind of exciting to see the box-office get reignited and to see consumers excited about what's available," said Kyle Davies, head of distribution for Overture.
Fans had a good range of choices among horror tales, action, family fare and romantic comedy, including the previous weekend's No. 1 movie, Universal's "Couples Retreat," which slipped to fourth-place with $17.9 million. "Couples Retreat" raised its 10-day total to $63.3 million.
"This is why the fall is such a great time to be not only a studio executive, but a moviegoer. It's really an eclectic mix out there. You don't get this in summer," said Paul Dergarabedian, box-office analyst for Hollywood.com.
Estimated ticket sales for Friday through Sunday at U.S. and Canadian theaters, according to Hollywood.com. Final figures will be released Monday.
1. "Where the Wild Things Are," $32.5 million.
2. "Law Abiding Citizen," $21.3 million.
3. "Paranormal Activity," $20.2 million.
4. "Couples Retreat," $17.9 million.
5. "The Stepfather," $12.3 million.
6. "Cloudy With a Chance of Meatballs," $8.1 million.
7. "Zombieland," $7.8 million.
8. "Toy Story" and "Toy Story 2" in 3-D, $3 million.
9. "Surrogates," $1.92 million.
10. "The Invention of Lying," $1.9 million.