Z for Zachariah is a post-apocalyptic science-fiction novel by Robert C. O'Brien that was published posthumously in 1974. The name Robert C. O'Brien was the pen name used by Robert Leslie Conly. After the author's death in 1973, his wife Sally M. Conly and daughter Jane Leslie Conly completed the book guided by his notes. Set in the USA, the story is in the form of a diary written from the first-person perspective of sixteen-year-old Ann Burden, who has survived a nuclear war and nerve gas through living in a small valley with a self-contained weather system.
According to Sally Conly, Z for Zachariah was her husband's second novel intended for adults, following his 1972 science-fiction thriller A Report from Group 17. O'Brien had previously established himself as a children's writer with his novels The Silver Crown (1968) and Mrs. Frisby and the Rats of NIMH (1971). In 1976, Z for Zachariah won the Jane Addams Children's Book Award and the Edgar Award for the best mystery fiction in the juvenile category.