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BACKGROUND: Ramadan fasting alters the timing and content of food, including dietary vitamin K, and medication intake, potentially affecting stability of vitamin K antagonist (VKA) treatment and clinical outcomes. OBJECTIVES: To assess population-level changes in VKA treatment quality and incidence of clinical events, including bleeding and venous and arterial thromboembolism, during Ramadan in the Netherlands. METHODS: Data from 17 Dutch anticoagulation clinics were linked to Statistics Netherlands. Prevalent VKA users in 2013-2019 with an immigration background who were likely to fast during Ramadan (ie, the Ramadan cohort) were studied from 2 months preceding Ramadan until 2 months after. During each 30-day interval, VKA treatment quality and risk of clinical events were assessed. A cohort of native Dutch VKA users was studied as negative control. RESULTS: The Ramadan cohort included 3835 VKA users (median age 65.8 years, 55.2% male). Frequency of international normalized ratio (INR) monitoring, INR variability, and time within target range remained similar across Ramadan. However, the proportion of supratherapeutic INRs was slightly higher during (18.9% ± 31.5% [mean ± SD]) and after Ramadan (month +1, 19.9% ± 32.7% and month +2, 19.7% ± 32.8%) compared with before (month -2, 18.1 ± 30.9% and month -1, 17.8 ± 31.5%). Meanwhile, there was a higher proportion of clinically relevant dose reductions during Ramadan (4.7%) than the other months (3.6%-4.3%). These were not observed in the native Dutch cohort ( = 139,207). Monthly risk of the composite of bleeding and thromboembolic events remained unchanged across Ramadan in both cohorts. CONCLUSION: There were no clinically relevant population-level changes across Ramadan in VKA treatment quality and clinical outcomes, except for a slightly higher proportion of supratherapeutic INRs and dose reductions.