Although prior studies have assessed lay perceptions of psychopathy, few studies have examined the effect of these perceptions on individuals’ legal judgments and decisions. The purpose of the present study was to assess lay perceptions of psychopathy and determine how these perceptions affect expert witness credibility ratings and sentencing decisions in a capital murder trial. Mock jurors completed an assessment of their perception of psychopathy, and subsequently, reviewed a case vignette of a capital murder trial. Approximately half of the mock jurors also read an excerpt of expert witness testimony concerning the defendant’s psychopathic traits. Mock jurors then sentenced the defendant, and if applicable, rated the expert witness’s credibility. Their perceptions of psychopathy were also reassessed. Mock jurors’ endorsement of psychopathy’s interpersonal and affective characteristics (e.g., conning, egotistical, remorseless) predicted their perceptions of the expert witness, such that stronger endorsement of these traits was associated with increased beliefs the expert witness’s testimony was credible and valid. Findings also indicated mock jurors whose stereotype of psychopathy included these traits were more likely to support a death penalty verdict. Additional findings, as well as implications for the inclusion of the psychopathy construct in the courtroom, are discussed.