Brace yourself if you're not prepared to feel old, but it's been more than two decades since everyone's favorite pint-sized detective hit the big screen.
The movie was a big milestone for fans of the classic book series, but also because it was the first film put out by the Nickelodeon Movies production company. But - aside from eating lots of mayo and tomato sandwiches - what have the cast been up to in the last 20 years?
Gregory Smith - Sport
Smith played one of Harriet's best friends, but you might know him better for leading roles in Small Soldiers and The Patriot. He also starred in the Disney Channel movie Zenon: Girl of the 21st Century.
As he moved into adulthood, Smith starred in TV's Everwood alongside Chris Pratt. More recently, you might have seen him on Rookie Blue. He's also building up his resume as a director on CW shows like Arrow and Legends of Tomorrow.
Dov Tiefenbach - The Boy with the Purple Socks
Tiefenbach had a handful of small TV roles before landing the mysterious part in this movie, but his career never really took off. He starred in Broadway shows and small movies, but his biggest part since Harriet was playing a drug dealer in Harold and Kumar.
Tiefenbach is also a musician, and was the front man for a "dirty indie pop" band called Theresa's Sound-World.
Charlotte Sullivan - Marion Hawthorne
Marion was a classic "mean girl" before the term was even invented, and Sullivan's performance earned her more parts on shows like Goosebumps, Are You Afraid of the Dark and The New Ghostwriter Mysteries.
After being cast as Marilyn Monroe on The Kennedys, Sullivan has been getting bigger roles on shows like Chicago Fire and Rookie Blue, where she played Gail Peck.
Robert Joy - Mr. Welsch
Joy already had big roles in movies like Waterworld, Radio Days and Desperately Seeking Susan before he played Harriet's dad. He went on to have a number of small parts in horror movies like Land of the Dead and The Hills Have Eyes.
If you recognize Joy from anything recent, it's probably for playing medical examiner Sid Hammerback on CSI: NY. But he still gets all kind of small parts on hit TV shows.
J. Smith-Cameron - Mrs. Welsch
Cameron has had an impressive acting career both on stage and on the big screen, but her most famous role was probably as Miss Sullivan in The First Wives Club.
She's mostly starred in small movies and plays since finishing Harriet the Spy, but you might recognize her as Melinda Mickens from True Blood.
Rosie O'Donnell - Ole Golly
Yes, the actress who played Harriet's nanny was already a household name for movies like A League of Their Own, Sleepless in Seattle and the Flintstones. Her talk show, The Rosie O'Donnell Show, was also launching just as the movie came out.
While she had a number of roles over the years, Rosie was most famous for hosting the daily chat show The View for six years. Now she stays busy with work on TV shows like Empire and The Fosters.
Vanessa Lee Carter - Janie
While playing one of Harriet's best friends was a big part, Carter's big break came right after this movie, when she starred in A Little Princess.
You may remember her as Jeff Goldblum's daughter from Jurassic Park: The Lost World. Carter also had parts in She's All That and 17 Again. While she goes by Vanessa Chester today, you can still see in in small TV roles from time to time.
Michelle Trachtenberg - Harriet
Playing the spunky sleuth was Trachtenberg's first big role, but she knocked it out of the park and followed up her success with more movie roles. After playing Dawn on Buffy the Vampire Slayer, she had leading roles in Inspector Gadget, Ice Princess, and 17 Again.
She made a whole new generation of fans after appearing on Gossip Girl as Georgina Sparks, but seems to have focused on starring in TV movies like The Christmas Gift in recent years.
Eartha Kitt - Agatha K. Plummer
The object of Harriet's investigation was played by none other than the legendary singer and actress. Kitt's decades-long career also included memorable roles as Catwoman on the 1960s Batman show, Madame Zeroni in Holes, and the evil Yzma in The Emperor's New Groove.
In the final two decades of her career, Kitt mainly focused on voice work, including playing Yzma and Vexus from My Life as a Teenage Robot. Kitt died on Christmas Day in 2008, from colon cancer. She was 81 years old.
Remember watching this movie when you were a kid?