Legitimacy theory describes how individuals evaluate an organization’s behavior, form propriety evaluations, and subsequently decide whether to publicly express their legitimacy judgments. These individual judgments are influenced by sources of collective validity, for example, from recognized authority or from peer endorsement. Although most research on this topic has focused on the effects of authority, we study the influence of peer endorsement on the public expression of legitimacy judgments. Additionally, we assess evaluators’ preparedness to expend cognitive effort, that is, their evaluative mode, as an important condition under which judgment expressions are made. We use a set of three vignette experiments and one online field study, all situated in social media that are quickly becoming the dominant setting for the expression of legitimacy judgments. This research provides new evidence that peer endorsement stimulates evaluators to express their judgments, particularly for evaluators who expend limited cognitive effort. Additionally, we find that evaluators in the active and passive evaluative modes act differently when their propriety evaluations are based on instrumental, moral, or relational considerations. These findings extend current legitimacy theory about how peer endorsement functions as a source of validity and when and how individual evaluators decide to publicly express their legitimacy judgments. This is important because individuals’ public expressions can bring about a cascade of judgments that change the consensus on an organization’s legitimacy, potentially contributing to institutional change.