In computer architecture, Gustafson's law (or Gustafson–Barsis's law) gives the speedup in the execution time of a task that theoretically gains from parallel computing, using a hypothetical run of the task on a single-core processor as the baseline. To put it another way, it is the theoretical slowdown of an already parallelized task if running on a serial processor. The law is named after computer scientist John L. Gustafson and his colleague Edwin H. Barsis, and was presented in the article Reevaluating Amdahl's Law in 1988.

In contrast to Amdahl's law, which assumes a fixed problem size and yields pessimistic scaling, Gustafson's law assumes that problem sizes grow with available computing resources, allowing far greater effective speedup from parallel execution.