This study examined the effect of message source credibility on e-cigarette harm perceptions among U.S. young adults. An online experimental study was conducted where young adults (n = 302, Mage = 23.7) were randomized to an e-cigarette public health education message from an expert or a peer young adult. Then, participants answered questions about their perceptions about the message source and e-cigarettes. Results suggest that young adults rated experts as a more credible source (vs. peer) (b = −0.39, SE = 0.15, 95% CI [−0.67, −0.10], p < 0.01). Young adults reported greater perceived credibility of the expert message (vs. peer), which was associated with increased e-cigarette harm perceptions. Increased perceived source credibility mediated the association of increased e-cigarette absolute harm perceptions from viewing an expert message (b = −0.11, SE = 0.04, 95% CI: −0.20, −0.02). Source credibility should be considered when designing e-cigarette education messages for young adults.