A distinct challenge of implicit self-esteem research is the dubious validity of measures for implicit self-esteem. We conducted two event-related potential (ERP) studies to examine whether implicit self-esteem measured by the Implicit Association Test (IAT) actually reflects the automatic self-evaluation. We adopted the regular IAT and the Go/No-go Association Task (GNAT), respectively, to measure implicit self-esteem in two studies. We focused on the P300 that reflects the stimulus evaluation process. Study 1 revealed that participants responded faster to self pairing with good than to self pairing with bad whereas self pairing with bad elicited a delayed P300 compared with self pairing with good. Study 2 replicated these findings and further highlighted that the index of implicit self-esteem based on reaction time is correlated with that based on P300 latency. Our results suggest that implicit self-esteem assessed by the IAT measures automatic self-evaluation, thus providing neural evidence for the validity of implicit self-esteem.