"A Sound of Thunder" is a science fiction short story by Ray Bradbury, first published in Collier's magazine in 1952. It was reprinted in his collections The Golden Apples of the Sun (1953), R is for Rocket (1962), The Stories of Ray Bradbury (1980), and A Sound of Thunder and Other Stories (2005). The Locus Index to Science Fiction Anthologies and Collections lists it as the first of the top ten most republished science fiction stories.

This well-known story about time travel revolves around a business called Time Safari, Inc. Time Safari promises to take people back in time so they can hunt prehistoric animals, such as Tyrannosaurus rex.

In order to avoid a time paradox, they are very careful to leave history undisturbed on the principle that even the slightest change can cause major changes in the future. Travelers are only allowed to shoot animals that are already about to die, and they are required to stay on a path which hovers slightly above the ground. Hunting trophies are not taken; no souvenir is allowed except a photograph of the hunter standing next to the dead monster.

In the story, a man leaves the floating path. Upon returning to the present, he notices subtle changes. English spelling is different, people and buildings are different, and the country appears to be more conservative politically (The story was published during Dwight Eisenhower's first presidential campaign). Looking at his boots, the man finds a crushed butterfly, apparently the cause of the changes.

The story is a fictional exploration of the Butterfly Effect (or "sensitive dependence upon initial conditions," in the words of Edward Lorenz) through the literary device of time travel. Interestingly, the story pre-dates the work of Edward Lorenz by nearly 10 years, long before the term was coined and the principles understood by the scientific community. The same effect occurs in planetary dynamics and was studied by Poincare in the 1900's, but not under its modern name. These subjects are grouped into the mathematical field of chaos theory.