Background: Little is known about patients who have caregiver proxies communicate with healthcare providers via portal secure messaging (SM). Since proxy portal use is often informal (e.g., sharing patient accounts), novel methods are needed to estimate the prevalence of proxy-authored SMs.; Objective: (1) Develop an algorithm to identify proxy-authored SMs, (2) apply this algorithm to estimate predicted proxy SM (PPSM) prevalence among patients with diabetes, and (3) explore patient characteristics associated with having PPSMs.; Design: Retrospective cohort study.; Participants: We examined 9856 patients from Diabetes Study of Northern California (DISTANCE) who sent ≥ 1 English-language SM to their primary care physician between July 1, 2006, and Dec. 31, 2015.; Main Measures: Using computational linguistics, we developed ProxyID, an algorithm that identifies phrases frequently found in registered proxy SMs. ProxyID was validated against blinded expert categorization of proxy status among an SM sample, then applied to identify PPSM prevalence across patients. We examined patients' sociodemographic and clinical characteristics according to PPSM penetrance, "none" (0%), "low" (≥ 0-50%), and "high" (≥ 50-100%).; Key Results: Only 2.3% of patients had ≥ 1 registered proxy-authored SM. ProxyID demonstrated moderate agreement with expert classification (Κ = 0.58); 45.7% of patients had PPSMs (40.2% low and 5.5% high). Patients with high percent PPSMs were older than those with low percent and no PPSMs (66.5 vs 57.4 vs 56.2 years, p < 0.001) had higher rates of limited English proficiency (16.1% vs 3.2% vs 3.5%, p < 0.05), lower self-reported health literacy (3.83 vs 4.43 vs 4.44, p < 0.001), and more comorbidities (Charlson index 3.78 vs 2.35 vs 2.18, p < 0.001).; Conclusions: Among patients with diabetes, informal proxy SM use is more common than registered use and prevalent among socially and medically vulnerable patients. Future research should explore whether proxy portal use improves patient and/or caregiver outcomes and consider policies that integrate caregivers in portal communication.