Research has demonstrated that secular individuals have fewer children, which limits secularization. This study asks two new questions: (1) Is there a country-level relationship between secularism and fertility? (2) If yes, does country-level secularism influence fertility independent of individual-level secularism? Using comparatively complete country-level data from multiple sources (N=181) and multilevel data from 55 countries in the World Values Survey (N=78,639), this study demonstrates a strong negative relationship between the proportion of nonreligious people in a country and both country-level and individual-level fertility. Secularism, even in small amounts, is associated with population stagnation or even decline, whereas highly religious countries have higher fertility. This country-level pattern is driven by more than aggregate lower fertility of individual nonreligious people. In fact, country-level secularism has a larger effect on religious people’s fertility. Beyond its importance for global religious demography, the societal-level association between secularism and fertility is relevant to key fertility theories and may help account, in part, for below-replacement fertility.