Alexithymia is the inability to identify and recognize emotions. The present study examined the impact of alexithymia on prolonged exposure (PE) therapy. Participants (n = 68) with PTSD underwent 10 PE sessions. Alexithymia was assessed via the Toronto Alexithymia Scale (TAS-20) and the emotional clarity and awareness subscales of the Difficulties in Emotion Regulation Scale. Treatment outcomes were assessed via the PTSD checklist and Clinician-Administered PTSD Scale for the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition at posttreatment and 6-month follow-up. Those with high alexithymia were more likely to endorse experiencing a full PTSD diagnosis immediately posttreatment, χ²(1) = 12.53, p = .002, and at 6-month follow-up, χ²(1) = 11.21, p = .004. Alexithymia was associated with delayed treatment effects on avoidance, with a significant reduction in symptomology observed from pre- to follow-up, t(51) = 4.52, p < .001, and not from pre- to posttreatment. Although both the low and high alexithymia groups showed significant changes in negative changes in thinking and mood, F(2, 14) = 9.18, p = .001, d = 1.57 and F(2, 50) = 13.86, p = .001, d = 1.49, respectively, the high alexithymia group exhibited a marginally lesser magnitude of treatment effect. Although those with significantly greater difficulties with emotional clarity were more likely to drop out of PE treatment, emotional clarity and awareness did not moderate treatment response. Our results confirm the efficacy of PE but also highlight that those with alexithymia show a delayed treatment response and may be at greater risk of pathology after treatment. (PsycInfo Database Record (c) 2024 APA, all rights reserved).