BACKGROUND: Diabetes is a complex, chronic disease that requires patients' effective self-management between clinical visits; this in turn relies on patient self-efficacy. The support of patient autonomy from healthcare providers is associated with better self-management and greater diabetes self-efficacy. Effective provider-patient secure messaging (SM) through patient portals may improve disease self-management and self-efficacy. SM that supports patients' sense of autonomy may mediate this effect by providing patients ready access to their health information and better communication with their clinical teams. OBJECTIVE: We examined the association between healthcare team-initiated SM and diabetes self-management and self-efficacy, and whether this association was mediated by patients' perceptions of autonomy support from their healthcare teams. DESIGN: We surveyed and analyzed content of messages sent to a sample of patients living with diabetes who use the SM feature on the VA's My HealtheVet patient portal. PARTICIPANTS: Four hundred forty-six veterans with type 2 diabetes who were sustained users of SM. MAIN MEASURES: Proactive (healthcare team-initiated) SM (0 or > /= 1 messages); perceived autonomy support; diabetes self-management; diabetes self-efficacy. KEY RESULTS: Patients who received at least one proactive SM from their clinical team were significantly more likely to engage in better diabetes self-management and report a higher sense of diabetes self-efficacy. This relationship was mediated by the patient's perception of autonomy support. The majority of proactive SM discussed scheduling, referrals, or other administrative content. Patients' responses to team-initiated communication promoted patient engagement in diabetes self-management behaviors. CONCLUSIONS: Perceived autonomy support is important for diabetes self-management and self-efficacy. Proactive communication from clinical teams to patients can help to foster a patient's sense of autonomy and encourage better diabetes self-management and self-efficacy.