We used electroencephalography (EEG) together with psychopharmacological stimulation to investigate the role of dopamine in neural oscillations during working memory (WM). Following a within-subjects design, healthy humans either received the dopamine precursor L-Dopa (150 mg) or a placebo before they performed a Sternberg WM paradigm. Here, sequences of sample images had to be memorized for a delay of 5 s in three different load conditions (two, four or six items). On the next day, long-term memory (LTM) for the images was tested. Behaviorally, L-Dopa improved WM and LTM performance as a function of WM load. More precisely, there was a specific drug effect in the four-load condition with faster reaction times to the probe in the WM task and higher corrected hit-rates in the LTM task. During the maintenance period, there was a linear and quadratic effect of WM load on power in the high theta (5–8 Hz) and alpha (9–14 Hz) frequency range at frontal sensors. Importantly, a drug by load interaction – mimicking the behavioral results – was found only in low theta power (2–4 Hz). As such, our results indicate a specific link between prefrontal low theta oscillations, dopaminergic neuromodulation during WM and subsequent LTM performance.