Background: Posttraumatic stress disorder (PTSD) is a common comorbid diagnosis in psychotic disorders. However, little is known about the effect of comorbid PTSD in youth at clinical high risk (CHR) for psychosis. Purpose: The purpose of this study was to investigate group differences in symptom severity, risk for psychotic conversion, and functioning among youth at CHR for psychosis with and without PTSD. Methods: This longitudinal study utilized data from N = 693 individuals who met criteria for CHR as part of the North American Prodrome Longitudinal Study 3 (NAPLS-3). Individuals who met criteria for PTSD were compared to those who did not meet criteria for PTSD on demographic, symptom and functional variables. Results: Individuals diagnosed with PTSD (N=54) reported more severe overall positive symptoms (p<0.001), negative symptoms (p<0.01), and general symptoms (p<0.001) compared to those without the diagnosis (N=639). These symptom differences were primarily accounted for by more grandiose (p<0.01) and perceptual symptoms (p<0.04), decreased experience of emotions (p<0.01) and occupational functioning (p<0.03), sleep disturbance (p<0.01), dysphoric mood (p<0.002) and impaired tolerance to stress (p<0.001) in those with PTSD. A diagnosis of PTSD was not associated with a greater rate of psychotic conversion or greater functional deficits. Conclusions: Our findings suggest that a diagnosis of PTSD is associated with more severe overlapping symptoms in individuals meeting the criteria for CHR but not psychotic conversion. Our findings underscore the importance of recognizing and treating comorbid psychiatric conditions including PTSD in CHR populations as a means of reducing symptoms and improving outcome.