Delaying gratification in value-based decision making is canonically related to activation in the dorsolateral prefrontal cortex (dlPFC), but past research neglected that the dlPFC is part of a larger frontoparietal network. It is therefore unknown whether the dlPFC causally implements delay of gratification in concert with posterior parts of the frontoparietal network rather than in isolation. Here, we addressed this gap by testing the effects of frontoparietal theta synchronization and desynchronization on impulsive decision making using transcranial alternating current stimulation (tACS). Healthy participants performed an intertemporal choice task and a 3-back working memory task while left frontal and parietal cortices were stimulated with a 5 Hz theta frequency at in-phase (synchronization), anti-phase (desynchronization), or sham tACS. We found frontoparietal in-phase theta tACS to improve working memory performance, while in the decision task anti-phase tACS was associated with more impulsive choices and stronger hyperbolic discounting of future rewards. Overall, our findings suggest that future-oriented decision making might causally rely on synchronous activation in a frontoparietal network related to working memory.