An important objection to the idea of public reason is that it permits and perhaps encourages citizens and public officials to give insincere justifications for their political decisions. Against this objection, I defend a principle of sincere public justification. First, I claim that political justifications must be public in two senses. They must be based on shared or public reasons, and those reasons must be presented in public discourse. Actual publicity, or the giving of reasons in public, is valuable for a number of reasons, but I focus mainly on its ability to improve the quality of political decisions. After defining the general concept of sincerity, and guarding against a certain form epistemic or psychological skepticism about it, I offer a principle of sincere public justification. I then defend that principle against two competing alternatives, a more demanding principle that includes a stringent motivational requirement, and a less demanding principle that abandons public sincerity in favor of private sincerity. Lastly, having stated and justified an ideal of public sincerity, I show how that ideal can be used to respond to the objection that public reason permits or encourages insincere political justification.