Background: Cancer survivors often experience cancer and treatment-related side effects and may also have comorbidities that may further impact their quality of life. Little is known about how cancer-related side effects and health status impact survivors’ beliefs and management of comorbidities. We undertook this study to assess the relationship between cancer-related side effects and diabetes (DM) self-management behaviors (SMB) and beliefs in cancer survivors with comorbid diabetes. Methods: We recruited diabetes patients recently diagnosed with early-stage breast or prostate cancer who were taking at least one oral hypoglycemic agent. Side effects for prostate cancer survivors were assessed using the Expanded Prostate Cancer Index Composite; side effects for breast cancer survivors were assessed using a standardized questionnaire for hormonal therapy-related side effects. Patients were asked to rate their overall health status and about the perceived impact of cancer treatment on DM. We measured DM medication adherence using the 8-item Morisky Medication Adherence Scale, and DM SMB were evaluated using the Summary of Diabetes Self-Care Activities. Descriptive and univariate analyses were used to assess factors associated with DM SMB. Results: Of the 52 patients recruited, 32 (62%) were male. Twenty-four (46%) patients reported cancer or treatment related side effects, with most (21, 88%) being moderately or severely bothered by these symptoms. Of these patients, 20 (83%) reported non-adherence to their DM medications and 18 (71%) reported checking their fingersticks < 5 times a week. Yet most patients (20, 84%) felt that their diabetes care was at least equally as important as their cancer care. Neither the presence nor severity of side effects was associated with DM SMB. However, patients who reported poorer overall health status were more likely to be worried about the impact of cancer treatment on diabetes management (75% vs. 25%, p < 0.01). Conclusions: Many cancer survivors report moderate to severe treatment-related side effects, although severity of side effects was not found to be associated with DM SMB. Further effort should focus on addressing survivors’ concerns about the impact of cancer treatment on comorbid disease management.