A traveling-wave reactor is a kind of nuclear reactor that can convert fertile material into nuclear fuel as it runs. Travelling wave reactors differ from other kinds of  reactors in their ability to use little or no enriched uranium; instead they burn fuel made from depleted uranium, spent fuel removed from light-water reactors, natural uranium, thorium, or some combination of these materials.

They are called "travelling wave" because fission does not take place in the entire reactor core but in a localized zone that advances through the core over time.

Unlike other reactors, travelling wave reactors can be fueled at the time of construction with enough depleted uranium to produce full power for 60 years or more. Depleted uranium, which is produced as a waste byproduct of the enrichment process, is widely available as a feedstock. Stockpiles in the United States alone currently contain approximately 700,000 tonnes of depleted uranium. It has been estimated that these stockpiles represent an energy resource equivalent to $100 trillion worth of electricity.

No traveling wave reactor has yet been constructed but, in 2006, TerraPower LLC, a company whose principal owner is Bill Gates, was established  to model and commercialize a practical travelling wave reactor. TerraPower has developed designs for low- to medium-power (300 megawatt) and large power (1000 megawatt) reactors.