The study aims to determine whether 60-minute sessions of prolonged exposure (PE) that include 20 minutes of imaginal exposure (IE) are noninferior to the standard 90-minute sessions that include 40 minutes of IE in treating posttraumatic stress disorder (PTSD) and to explore the relationship of treatment outcome to within- and between-session habituation and change in negative cognitions. Thirty-nine adult veterans with chronic PTSD were randomly assigned to 90-minute (n = 19) or 60-minute (n = 20) sessions of PE. PTSD symptoms were assessed by an unaware independent evaluator before and after treatment and at 6-month follow-up. Self-reports of depression and negative cognitions were assessed before and after treatment. Participants in both conditions showed significant reductions in PTSD symptoms. Sixty-minute sessions were found to be noninferior to 90-minute sessions in reducing PTSD symptoms, as the upper bound of the 95% confidence interval for the difference between conditions in the PTSD Symptom Scale–Interview (posttreatment: 6.00; follow-up: 6.77) was below the predefined noninferiority margin (7.00). Participants receiving shorter sessions showed less within- and between-session habituation than those receiving longer sessions, but no group differences in reductions in negative cognitions were found. The current findings indicate that the outcomes of 60-minute sessions of PE do not differ from those of 90-minute sessions. In addition, change in trauma-related cognitions and between-session habituation are both potential mechanisms of PE.