One crucible for theories of religion is their ability to predict and explain the patterns of belief and disbelief. Yet, religious nonbelief is often heavily stigmatized, potentially leading many atheists to refrain from outing themselves even in anonymous polls. We used the unmatched count technique and Bayesian estimation to indirectly estimate atheist prevalence in two nationally representative samples of 2,000 U.S. adults apiece. Widely cited telephone polls (e.g., Gallup, Pew) suggest U.S. atheist prevalence of only 3–11%. In contrast, our most credible indirect estimate is 26% (albeit with considerable estimate and method uncertainty). Our data and model predict that atheist prevalence exceeds 11% with greater than .99 probability and exceeds 20% with roughly .8 probability. Prevalence estimates of 11% were even less credible than estimates of 40%, and all intermediate estimates were more credible. Some popular theoretical approaches to religious cognition may require heavy revision to accommodate actual levels of religious disbelief.