IMPORTANCE Caregiver strain has been shown to be associated with adverse effects on caregivers' health, particularly among those with cardiovascular disease. Less is known about the association of caregiver strain with health behaviors among caregivers with diabetes, a disease that requires a high degree of self-care. OBJECTIVE To examine the association between caregiver strain and diabetes self-care among caregivers with diabetes. DESIGN, SETTING, AND PARTICIPANTS This cohort study was conducted between July 13, 2018, and June 25, 2020, using data on 795 US caregivers aged 45 years or older with self-reported diabetes from the Reasons for Geographic and Racial Differences in Stroke (REGARDS) study, which comprised 30 239 Black and White adults 45 years or older throughout the US enrolled from January 2003 to October 2007. EXPOSURES Caregiver strain, assessed by self-report in response to the question, "How much of a mental or emotional strain is it to provide this care?" Response options were no strain, some strain, or a lot of (high) strain. MAIN OUTCOMES AND MEASURES Diabetes self-care, which was assessed across 4 domains (Mediterranean diet adherence, physical activity, smoking status, and medication adherence), and a composite self-care score summing performance across these domains. The association between caregiver strain and diabetes self-care was examined with multivariable Poisson regression adjusting for demographic, clinical, physical and mental functioning, and caregiving covariates. RESULTS Among the 795 caregivers with diabetes included in the study, the mean (SD) age was 63.7 (8.6) years, 469 (59.0%) were women, and 452 (56.9%) were Black individuals. Overall, 146 caregivers (18.4%) reported high caregiver strain. In unadjusted models, high caregiver strain was associated with less physical activity (prevalence ratio [PR], 0.66; 95% CI, 0.45-0.97), low medication adherence (PR, 0.80; 95% CI, 0.68-0.94), and worse self-care (PR, 0.65; 95% CI, 0.44-0.98). In adjusted models, the association between some and high caregiving strain with low medication adherence remained significant (adjusted PR: some strain, 0.88 [95% CI, 0.78-0.99]; high strain, 0.83 [95% CI, 0.69-0.99]). CONCLUSIONS AND RELEVANCE In this cohort study of US adult caregivers with diabetes, a high level of strain was associated with low medication adherence. Increased awareness of the prevalence of caregiver strain and potential ramifications on caregivers' self-care appears to be warranted among health care professionals and caregivers.