Third-party quality assurance seals have emerged as a prominent mechanism to reduce uncertainty and increase purchase conversion in online markets. However, systematic studies of the effectiveness of these seals are scarce. In this study, we exploit a unique data set of 9,098 shopping sessions at an online retailer's website to empirically measure the value and effectiveness of assurance seals on the likelihood of purchase by shoppers. The data set is collected from a randomized field experiment conducted by a large seal provider, which enables us to infer the causal impacts of the presence of an assurance seal. We find strong evidence that the presence of the assurance seal increases the likelihood of purchase conversion. We discuss the implications of our findings for online retailers, third-party certifiers, policymakers, and researchers.