Keeper of the Lost Cities is an upper-middle-grade fantasy series by Shannon Messenger that has appeared on the New York Times Best Seller list for a total of ten weeks.
The protagonist of the series, Sophie, is a twelve-year-old elf with telepathy who lives in San Diego, California with a human family.
In the first book, Sophie, while on a school field trip, is tracked down by a teenage elf, Fitz, who reveals that she is an elf. He discusses the reality of their world with Sophie and tells her that she has to move to the Lost Cities—a place unknown to humankind where elves, trolls, ogres, goblins, gnomes and dwarves live in harmony. She is forced to leave her human family behind and stay with an elvin couple, Grady and Edaline, who later adopt her. She starts school at a noble elvin academy named Foxfire.
The main conflict in the series is between the Black Swan, a rebel organization, and the Neverseen, a group of criminal extremists. Sophie learns that the Black Swan are her creators, and that her old neighbor from the human world, Mr. Forkle, is the director of the Black Swan's Project Moonlark, in which she was created. Her biological parents are unknown due to catastrophes that could be created if their identities are discovered.
The series currently consists of nine novels and two novellas, Unlocked and Unraveled.
A graphic novel adaptation to Keeper, adapted by Celina Frenn and illustrated by Gabriella Chianello, was released on November 7, 2023, covering half of the first book. Part 2, illustrated by Valeria Tenaga Romanazzi, covered the remainder of the novel and released on December 2, 2025.
The series has a largely positive reception. Kirkus Reviews said that the first book was "wholesome shading to bland, but well-stocked with exotic creatures and locales." Booklist Reviews called it an "exciting start to a promising series."
On January 11, 2021, it was announced that Ben Affleck would be directing an adaptation of Keeper of the Lost Cities for Walt Disney Pictures.
On April 18, 2024, it was announced that Warner Bros. Pictures had landed the rights to the Keeper of the Lost Cities adaptation, with Emma Watts serving as producer and Messenger serving as executive producer.