Previous research has implicated the prefrontal cortex (PFC) in successful associative encoding and subjective awareness of one’s memory performance. We tested the causal role of the PFC in these processes by applying transcranial direct current stimulation (tDCS) during a verbal associative encoding and judgment-of-learning (JOL) task. tDCS over the PFC impaired associative encoding compared to sham and parietal tDCS, as shown by fewer hits on a subsequent associative recognition test. There were no effects of tDCS on the magnitude or accuracy of JOLs. These results suggest the PFC plays a causal role in objective measures of encoding, and that objective and subjective aspects of associative encoding are separable.