Fitzhugh wrote two sequels to the book: The Long Secret (1965) and Sport (1979, published posthumously). Both books received mixed reviews. In 2002, a sequel Harriet Spies Again appeared; written by Helen Ericson, it also received mixed reviews. Another sequel, Harriet the Spy, Double Agent by Maya Gold, was published in 2005; one review of that book stated "there's not much to interest readers here."
- Harriet the Spy (Harper & Row, 1964); also Harriet, the Spy
- The Long Secret (Harper & Row, 1965)
- Sport (Dell Publishing/Delacorte Press, 1979), Fitzhugh
- Harriet Spies Again (Dell/Delacorte, 2002), Helen Ericson [and Fitzhugh]
- Harriet the Spy, DoubleAgent (Dell/Delacorte, 2005), Maya Gold and Fitzhugh