Background Most patients with posttraumatic stress disorder (PTSD) suffer from sleep problems. Concerns have been raised about possible detrimental effects of sleep problems on the efficacy of psychological treatments for PTSD. In this study, we investigated the relation of session‐to‐session changes in PTSD symptoms and sleep, and tested whether sleep problems predicted poorer short‐ and long‐term treatment outcome. Methods Self‐reported sleep quality, sleep duration, and PTSD symptoms were assessed weekly in a consecutive sample of 246 patients who received cognitive therapy for PTSD (CT‐PTSD; Ehlers & Clark, 2000), and at follow‐up (mean = 247 days posttreatment). Additionally, moderating effects of medication use and comorbid depression were assessed. Results Sleep and PTSD symptoms improved in parallel. The relation was moderated by depression: Sleep problems at the start of therapy did not predict improvement in PTSD symptoms during treatment for patients without comorbid depression. Patients with comorbid depression, however, showed less rapid decreases in PTSD symptoms, but comparable overall outcome, if their sleep quality was poor. Residual sleep problems at the end of treatment did not predict PTSD symptoms at follow‐up once residual PTSD symptoms were taken into account. Conclusions CT‐PTSD leads to simultaneous improvement in sleep and PTSD symptoms. Sleep problems may reduce the speed of recovery in PTSD patients with comorbid depression. For these patients, additional treatment sessions are indicated to achieve comparable outcomes, and additional interventions targeting sleep may be beneficial. For those without comorbid depression, self‐reported sleep problems did not interfere with response to trauma‐focused psychological treatment.