Background <p>Although epidemiological studies have established a link between diabetes and frailty, the dynamic changes in frailty before and after diabetes diagnosis remained inadequately characterized. This study aimed to examine the trajectories of frailty preceding and following diabetes diagnosis in older adults and to identify potential modifiers of the association between diabetes and trajectories of frailty.</p> Methods <p>We included adults aged≥65 years without baseline diabetes from the Chinese Longitudinal Healthy Longevity Survey (2002–2018). Incident diabetes was defined by self-reported physician diagnosis or diabetes-related hospitalization. The date of diagnosis was estimated as the midpoint between the last non-diabetic interview and the first report of diabetes. Frailty was assessed repeatedly using a 43-item frailty index (FI). Linear mixed-effects models were used to compare frailty trajectories before and after diabetes onset.</p> Results <p>Among 9,992 participants (mean age 82.12 years; 51.31% women), 420 developed diabetes over a median follow-up of 8.67 years. After diagnosis, the rate of change in FI increased significantly, with an annual additional increase of 0.52 (95% CI: 0.33, 0.71). The most pronounced acceleration was observed in instrumental activities of daily living (β = 0.80/year, 95% CI: 0.27, 1.32). Subgroup analyses revealed stronger associations in participants aged ≥80 years (β = 1.01/year, 95% CI: 0.42, 1.59), those who were single (β = 0.60/year, 95% CI: 0.24, 0.95), and those with BMI &lt;22 kg/m² (β = 0.72/year, 95% CI: 0.41, 1.03).</p> Conclusions <p>Incident diabetes was associated with accelerated progression of frailty in older adults, particularly in the domain of instrumental activities of daily living. Regular functional assessments and targeted interventions following diabetes diagnosis was important, especially for high-risk subgroups including the oldest-old, unmarried individuals, and those with lower BMI.</p>