Observers pervasively witness organizational justice. Although prior research, including meta-analyses, has examined observer responses to injustice, existing work has neither differentiated across distinct facets of observed injustice nor compared the relative strength of competing mediating mechanisms. This meta-analysis investigates the effects of observed injustice in the workplace by addressing four key questions: (a) how observed injustice shapes observer responses, including emotional (i.e., anger, empathy, guilt, positive affect), cognitive (i.e., moral perception and moral judgment), relational (i.e., exchange quality with organizations and coworkers), psychological strain (i.e., burnout), behavioral (i.e., citizenship and deviance behaviors), and employee outcomes (i.e., attitudes, performance, and well-being); (b) whether the effects vary across different facets of observed injustice—overall, distributive, interpersonal, and procedural injustice; (c) which competing mechanisms underlie observer responses and outcomes, with a focus on helping victims, punishing perpetrators, task performance, and withdrawal behaviors; and (d) how cultural dimensions (e.g., individualism-collectivism, power distance) moderate the prominence of these mechanisms. We first identify six prominent mechanisms: anger, exchange quality, empathy, guilt, negative affect, and justice perception. Synthesizing evidence from 222 unique samples (N = 96,408 participants), we compare the relative strength of these mechanisms. Our findings show that when all six mechanisms are considered simultaneously, exchange quality, anger, and justice perception consistently predict observer behaviors. Specifically, observed injustice primarily influences helping and punishing behaviors through anger, task performance via exchange quality and negative affect, and withdrawal behaviors through exchange quality and anger. Theoretical and managerial implications are discussed.