Transient episodes of brain oscillations are a common feature of both the waking and the sleeping brain. Sleep spindles represent a prominent example of a poorly understood transient brain oscillation that is impaired in disorders such as Alzheimer’s disease and schizophrenia. However, the causal role of these bouts of thalamo-cortical oscillations remains unknown. Demonstrating a functional role of sleep spindles in cognitive processes has, so far, been hindered by the lack of a tool to target transient brain oscillations in real time. Here, we show, for the first time, selective enhancement of sleep spindles with non-invasive brain stimulation in humans. We developed a system that detects sleep spindles in real time and applies oscillatory stimulation. Our stimulation selectively enhanced spindle activity as determined by increased sigma activity after transcranial alternating current stimulation (tACS) application. This targeted modulation caused significant enhancement of motor memory consolidation that correlated with the stimulation-induced change in fast spindle activity. Strikingly, we found a similar correlation between motor memory and spindle characteristics during the sham night for the same spindle frequencies and electrode locations. Therefore, our results directly demonstrate a functional relationship between oscillatory spindle activity and cognition. •Feedback-controlled tACS (FB-tACS, 12 Hz) boosted subsequent sleep spindle activity•FB-tACS enhanced sleep-dependent motor, but not declarative memory, consolidation•Stimulation-induced fast spindle activity changes predicted motor memory benefits•The correlation of spindles and motor memory in sham session agrees with FB-tACS results Lustenberger et al. engineered a novel feedback-controlled spindle stimulation approach that selectively targeted and modulated sleep spindles in real time. This approach revealed, for the first time, that fast sleep spindles play a functional role in motor memory consolidation.