Free play is unstructured play among children without adult supervision who decide themselves how and what to play and make up the rules as they go along. Free play is crucial for child development, and promotes social skills, emotional health, resilience, cooperation, confidence, cognitive growth, and brain development.

A lack of free play has negative effects in childhood and through adolescence and beyond, but it isn't clear how serious the consequences are. Declines in unstructured outdoor play among children in the last few decades has led to concern among experts about negative physical and mental effects in many countries including increased obesity, depression, anxiety, and other issues.

Mark Twain, in the voice of Tom Sawyer (1876), defined play in contrast with work: "Work consists of whatever a body is obliged to do, and play consists of whatever a body is not obliged to do."

Peter Gray defined free play as unstructured play among children without adult supervision "in which the players themselves decide what and how to play and are free to modify the goals and rules as they go along. Pickup baseball is free play; a Little League game is not."

Ellen Greenlaw defines it as "any type of unstructured play that is directed by the child".

Free play is play that isn’t organized or directed by adults or older peers and that generally doesn't have a defined purpose or outcome.

Free play has been termed "self-directed", as opposed to play which is guided by adults. An earlier term for this is unstructured play.

The term free play was used in its current sense in the 1967 educational film Organizing Free Play, produced by Vassar College for project Head Start (program) training programs.