A feature film based on the novel starring Michelle Trachtenberg was released by Nickelodeon Movies in 1996.

Eleven-year-old Harriet M. Welsch is an aspiring writer who lives in New York City's Upper East Side.

Encouraged by her nanny, "Ole Golly", Harriet observes others and writes her thoughts down in a notebook as practice for her future career.

Harriet's best friends are Simon "Sport" Rocque, a serious boy who wants to be a certified public accountant or a ball player, and Janie Gibbs, who wants to be a scientist.

Harriet's life changes abruptly when Ole Golly's suitor, Mr. Waldenstein, proposes and she accepts.

The students form a "Spy Catcher Club" in which they think up ways to make Harriet's life miserable, such as stealing her lunch and passing nasty notes about her in class.

[7] One 1965 reviewer called the book "a brilliantly written, unsparing realistic story, a superb portrait of an extraordinary child".

[8] Another reviewer found that it "captures the feelings, thoughts and situations of a modern city child with remarkable clarity and dimension".

"[14] Whitney Matheson wrote on the USA Today site in 2002 that Harriet "attracts dedicated, lifelong supporters".

[16] In 2005, the ex-CIA officer Lindsay Moran cited the Harriet the Spy series of books as an inspiration for her career.

[18] In 2012, Harriet the Spy was ranked number 17 among all-time children's novels in a survey published by School Library Journal.

Charlotte, an absentee mother who has been living well abroad in Europe most of the time, returns to New York City upon hearing of her father's illness, scheming to increase her share of Simon's inheritance by kidnapping Sport and imprisoning him in the Plaza Hotel for a week.

[29][30][31] Another sequel, Harriet the Spy, Double Agent by Maya Gold, was published in 2005;[32][33] one review of that book stated "there's not much to interest readers here.

[39][40] In March 2010, Disney Channel aired a version of the story, Harriet the Spy: Blog Wars.