Background: Cancer and diabetes mellitus (DM) are leading causes of death in the US and survivors with comorbid DM have a significantly higher mortality. Furthermore, posttraumatic stress disorder (PTSD) can be triggered by life-threatening illnesses such as cancer; however, little is known about the impact of cancer-related PTSD (Ca-PTSD) on illness beliefs and glycemic control. We undertook this study to assess factors associated with DM management in breast cancer survivors with DM. Methods: We recruited women with Stage 0-IIIA breast cancer diagnosed in the past 10 years, who had completed chemotherapy and/or hormonal therapy and had pre-existing DM on ≥1 oral DM medication. The Impact of Event Scale-Revised (IES-R) was used to assess distress caused by cancer. A total IES-R score of ≥33 identified those with probable Ca-PTSD. Cancer and DM related beliefs were assessed using the Illness Perception Questionnaire. Objective adherence to DM medication was assessed using electronic caps that record bottle openings. Wilcoxon rank-sum tests were used to examine the relationship between Ca-PTSD and illness beliefs. Pearson correlation tests were used to assess the relationship between cancer related stress, DM medication adherence, and glycemic control. Results: Of 111 women recruited to date, mean age was 65.4 years and 37% were black, 31% white, 14% Hispanic, 6% Asian, and 8% other. Of these, 15% had probable Ca-PTSD. Survivors with Ca-PTSD were more likely to believe the consequences of cancer and DM were more severe (median score 23.5 vs. 19.0 [p < 0.001] and 24.0 vs. 16.0 [p < 0.001], respectively), that cancer and DM were more cyclical or recurrent (15.0 vs. 9.0 [p = 0.002] and 13.0 vs. 8.5 [p = 0.002], respectively), and there was greater emotional impact due to cancer and DM (24.0 vs. 16.5 [p < 0.001] and 23.0 vs. 14.0 [p < 0.001], respectively). Additionally, those with Ca-PTSD were less likely to understand their cancer (14.0 vs. 20.0, p = 0.004). Furthermore, as cancer-related stress scores increased, glycemic control worsened (r = 0.23, p = 0.03) and DM medication adherence decreased (r = -0.23, p = 0.04). Conclusions: Breast cancer survivors with comorbid DM who have Ca-PTSD are more likely to have negative beliefs about both cancer and DM. Cancer-related stress is also associated with poorer glycemic control and DM medication adherence. Providers should be aware of the impact of cancer related stress on beliefs about comorbid illnesses and increase interventions that support cancer survivors to better manage their comorbidities.