Background Low 24 h urinary potassium excretion, reflecting low potassium intake, is associated with premature mortality in the general population. Objectives Whether urinary potassium excretion is associated with all-cause mortality in patients with type 2 diabetes is unknown. Methods We performed a prospective cohort study in 654 patients with type 2 diabetes of the Diabetes and Lifestyle Cohort Twente (DIALECT). Sex-specific tertiles of 24 h urinary potassium excretion were analysed in a multivariable Cox regression model with all-cause mortality. The outpatient program of the hospital uses a continuous surveillance system by the municipal registry of death to ensure up-to-date information on the patient's status (alive or deceased). Food frequency questionnaires were used to study associations between urinary potassium excretion and food products. Results Urinary potassium excretion at baseline was 84 ± 25 mmol·day−1 in males and 65 ± 22 mmol·day−1 in females, corresponding to estimated potassium intakes of 4250 ± 1270 mg·day−1 and 3300 ± 875 mg·day−1. During median follow-up for 5.2 [interquartile range 2.7 − 7.9] years, 96 participants died. In a fully adjusted model, patients in the lowest sex-specific tertile had a higher risk of all-cause mortality, compared with patients in the highest sex-specific tertile (HR: 2.09; 95% CI: 1.06, 4.10; P = 0.03). Patients in the lowest sex-specific tertile consumed less fruits and vegetables, dairy, coffee, and potato products compared with patients in the highest sex-specific tertile (all P < 0.05). Conclusions Low potassium intake is associated with a higher risk of all-cause mortality in Dutch patients with type 2 diabetes. Intervention studies are needed to determine whether potassium supplementation improves longevity in patients with type 2 diabetes.