Rhoda Penmark is a fictional character in William March's 1954 novel The Bad Seed and the stage play of the same name adapted from it by Maxwell Anderson.

While at a school picnic, Rhoda murders her classmate, Claude Daigle, who won a special penmanship award that she feels she deserved.

Christine, who has always vaguely sensed something wrong with her daughter, is troubled, but dismisses any possibility that Rhoda was actually involved in the boy's death.

Only two adults see through Rhoda's charm — Leroy, the mean-spirited janitor at the Penmarks' apartment building; and, to a lesser extent, her teacher Miss Fern, who observes that she is a poor loser and rather selfish.

Once, Rhoda was even expelled from a school for repeatedly being caught lying to teachers and staff, who described her as a "cold, self-sufficient child who plays by her own rules".

Christine tries to relieve her fears by talking abstractly about the murder with her adopted father and Mrs. Breedlove, a neighbor who dabbles in psychiatric theories about personality.

During the conversation, she recovers a long-repressed memory of her real mother, "the incomparable Bessie Denker", a serial poisoner who died in the electric chair.

In the 2018 remake starring and directed by Rob Lowe, the character Emma Grossman, portrayed by Mckenna Grace, is based on Rhoda Penmark.

Brian Solomon writes, "The grand-mammy of all messed-up horror movie kids, Rhoda Penmark is a pint-sized terror of biblical proportions.