Objective Posttraumatic stress symptoms (PTSS) can be triggered by a diagnosis of a potentially life‐threatening illness such as cancer. Little is known about the impact of cancer‐related PTSS symptoms on self‐management behaviors for comorbid chronic medical conditions such as diabetes mellitus (DM). Methods We recruited patients with DM and a recent diagnosis of early‐stage cancer from two medical centers in New York City. Cancer‐related PTSS were assessed using the Impact of Events scale (IES, score ≥26). DM self‐management behaviors (medication adherence, exercise, healthy diet, and glucose testing) were measured 3 months later. Logistic regression was used to assess the association between cancer‐related PTSS symptoms and DM self‐management behaviors, adjusting for gender, marital status, and anxiety symptoms. Results Of 56 participants recruited, 33% reported cancer‐related PTSS symptoms. Elevated cancer‐related PTSS symptoms were associated with lack of healthy diet (Odds Ratio [OR]: 0.08, 95% confidence interval [CI]: 0.01‐0.62). Conclusions Early‐stage cancer survivors with cancer‐related PTSS symptoms were less likely to adhere to some DM self‐management behaviors. Providers should recognize the impact of cancer‐related PTSS symptoms to better support comorbid disease management in cancer survivors.