Does understanding how U.S. Supreme Court justices actually decide cases undermine the institutional legitimacy of the nation's highest court? To the extent that ordinary people recognize that the justices are deciding legal disputes on the basis of their own ideological biases and preferences (legal realism and the attitudinal model), the belief that the justices merely “apply” the law (mechanical jurisprudence and the myth of legality) is difficult to sustain. Although it is easy to see how the legitimacy of the Supreme Court, the most unaccountable of all American political institutions, is nurtured by the view that judicial decisionmaking is discretionless and mechanical, the sources of institutional legitimacy under legal realism are less obvious. Here, we demonstrate, using a nationally representative sample, that the American people understand judicial decisionmaking in realistic terms, that they extend legitimacy to the Supreme Court, and they do so under the belief that judges exercise their discretion in a principled and sincere fashion. Belief in mechanical jurisprudence is therefore not a necessary underpinning of judicial legitimacy; belief in legal realism is not incompatible with legitimacy.