<p>Neural variability, i.e. random fluctuations in neural activity, is a ubiquitous and sizable brain feature that impacts behavior. Its functional role however remains unclear and neural variability is commonly viewed as a nuisance factor degrading behavioral efficiency. Using functional magnetic resonance imaging in humans and computational modeling, we show here that neural variability provides a solution to the open issue regarding how the brain produces efficient adaptive behavior in uncertain and changing environments without facing computational complexity problems. We found that neural variability in the medial prefrontal cortex (mPFC) enables decision-making processes in the mPFC to produce near-optimal behavior in uncertain and ever-changing environments without involving complex computations known in such environments to rapidly become computationally intractable. The results thus suggest that in the same way as genetic variability contributes to adaptive evolution, neural variability contributes to efficient adaptive behavior in real-life environments.</p>