Self-concept is linked to student achievement in many domains. In this study, we examined reading self-concept's (RSC) and RSC calibration accuracy's links to reading achievement across different contexts via multi-level analyses of 34 countries' 158,848 fifteen-year-olds' reading tests and questionnaire responses. Students with higher RSC, higher calibration accuracy (of RSC to their reading scores) or underconfidence (relative to their reading scores) had higher reading scores. RSC was more strongly linked to reading scores in countries that were richer, less equal, more collective, less uncertainty averse, less hierarchical, or less rigid regarding gender roles. Calibration accuracy was also more strongly linked to reading achievement in more hierarchical, individualistic, or uncertainty-tolerant countries. In more individualistic countries, underconfident students were more likely to have above average reading achievement. Hence, excessive confidence does not necessarily benefit students, especially in more individualistic countries. (Contains 2 figures and 11 tables.)