In a sample of 455 Taiwanese upper-secondary school students, latent variable structural equation modeling was used to test hypothesized relationships between beliefs about knowledge in science, beliefs about justification for knowing in science, and justification of knowledge claims concerning science encountered on the Internet. Results indicated that participants displayed adaptive beliefs in the tentative and evolving nature of scientific knowledge while their beliefs about justification for knowing in science seemed somewhat less adaptive. Further, their self-reports of Internet-specific justification of knowledge claims suggested that they did not strongly believe that science information posted on the Internet needed to be carefully evaluated. The structural equation modeling indicated that beliefs in the tentative and evolving nature of scientific knowledge had direct positive relationships with beliefs in justification by research-based authority and justification by multiple sources in science, which, in turn, mediated the relationships between beliefs in tentative and evolving knowledge and adaptive beliefs about Internet-specific justification of knowledge claims. Beliefs in the unambiguous, certain nature of scientific knowledge had a direct positive relationship with beliefs in justification by school-based authority (i.e., the science teacher and the science textbook), but beliefs in justification by school-based authority were not related to Internet-specific justification beliefs. We highlight the unique contributions of this study to the field of epistemic belief and science education research, and its theoretical and educational implications are discussed.