The Pigman is widely acknowledged as a turning point in young adult literature. According to Jack Davis Forman in Presenting Paul Zindel, Zindel's "commitment to write realistically about the concerns of teenagers" set his books apart from "the previous genre of teen fiction calcified in the gender and age stereotypes of the 1950s." Forman quoted Kenneth Donelson and Alleen Nilssen, whose survey, Literature for Today's Young Adults, noted that The Pigman "established a new type of adolescent fiction in which teenagers dealing with interpersonal or societal problems were depicted with candor and seriousness."
As Forman noted, previous books had portrayed teenagers as adults wished they were, or thought they should be, and were "pedestrian, predictable, and formulaic." Zindel was one of the first writers to show teenagers from a teenage point of view, unfiltered by adult notions of right, wrong, or what their behavior should be. According to Forman, a reviewer in Horn Book called The Pigman "a now book," and commented that few books were "as cruelly truthful about the human condition." Forman also noted that a New York Times reviewer wrote that the book had "the right combination of the preposterous and the sensible," but commented that Zindel's overt explanation of the book's "moral" was patronizing to readers. Forman also quoted Publishers Weekly reviewer Lavinia Russ, who remarked on her excitement at discovering such a skilled new writer by saying she felt "like the watcher of the skies when a new planet swam into its ken."
In English Journal, Loretta Clarke praised the book, except for the ending; like the New York Times reviewer, she felt that the last three lines were weak:
They build their own cages, we could almost hear the Pigman whisper, as he took his children with him.
"These three lines intrude upon the story," Clarke wrote, but commented that otherwise, Zindel "has reflected through his adolescent writers an adolescent view of life."
In Teacher Librarian, Teri Lesesne wrote that the book was "one of those touchstone books that set apart novels for adolescents," that it "set the standard for writers to follow," and that it "is considered by many to be the first truly YA [young adult] book."
The book was listed as one of the Child Study Association of America's Children's Books of the Year for 1968, and won the Boston Globe-Horn Book Award in 1969. It was also listed as one of the American Library Association's Best Young Adult Books in 1975.