OBJECTIVES: To investigate the association between chronic kidney disease (CKD) and dementia diagnosis in a real-world primary care setting in England. DESIGN: Matched cohort study. SETTINGS: English primary care in the Clinical Practice Research Datalink. PARTICIPANTS: People aged ≥18 years with predialysis CKD (stages 3-5, defined as two measurements of estimated glomerular filtration rate <60 mL/min/1.73 m2 for 3 months) from 2004 to 2014, and people without known CKD who were matched on age, sex, general practice and calendar time in a 1:1 ratio. PRIMARY AND SECONDARY OUTCOME MEASURES: First-ever diagnosis of dementia recorded by GPs. We also examined all-cause death as a secondary outcome to discuss potential competing risk of mortality in the association between CKD and dementia diagnosis. RESULTS: In a matched cohort of 242 349 pairs with and without CKD (mean age 75.4±9.7 years, 39.3% male), the crude incidence rate of dementia diagnosis was 11.4/1000 and 9.4/1000 person-years, respectively. There was an association between CKD status and incident dementia diagnosis in the first 6 months of the follow-up (adjusted rate ratio (aRR) 1.58, 95% CI 1.44 to 1.74), which attenuated after 6 months (aRR 1.12, 95% CI 1.08 to 1.16). Among patients with CKD, there was no evidence of association between CKD stage and incident dementia diagnosis; compared with stage 3a, aRR (95% CI) was 1.04 (0.91 to 1.18) for stage 3b and 0.94 (0.74 to 1.20) for stages 4 or 5 in the first 6 months, and 0.97 (0.92 to 1.01) and 0.89 (0.80 to 0.98) thereafter. We found a strong association between worsening CKD stage and all-cause mortality. CONCLUSION: We identified a co-occurrence of detection of CKD and dementia in real-world clinical practice and a strong competing risk of mortality in the association between CKD stage and dementia, while a weak association between CKD status and dementia was suggested in the long term.