Diet has been shown to be associated with rheumatoid arthritis (RA), and magnesium has been shown to inhibit inflammatory responses, but research on the relationship between dietary magnesium and RA is limited and controversial. In this study, we aimed to explore the non-linear relationship between dietary magnesium intake and RA in US women. Cross-sectional survey. National Health and Nutrition Examination Survey (NHANES). Non-linear relationship between dietary magnesium intake and prevalence of RA. A total of 13 324 women aged 18-80 years (RA n=12 306, non-RA n=1018) were included in this study. Overall, the absolute risk (AR) of RA was 7.24% in all participants. In the multivariable logistic regression analysis, we found a negative correlation between dietary magnesium intake and RA (OR=0.84, 95% CI 0.75 to 0.95, p=0.006). When we converted dietary magnesium intake into a categorical variable (tertiles), the ARs of the low group, the middle group and the high group were 9%, 7.1% and 4.9%, respectively. We noticed that the ORs between the three groups were not equidistant; then, we detected a U-shaped linking by smooth curve fitting and obtained inflection points at 181 and 446 mg/day. The prevalence of RA decreased when dietary magnesium intake was <181 mg/day (OR=0.7, 95% CI 0.5 to 0.8, p<0.001) and increased when it was >446 mg/day (OR=2.8, 95% CI 1.2 to 6.6, p=0.020), remaining at a minimum when it was between 181 and 446 mg/day (OR=1.0, 95% CI 0.7 to 1.2, p=0.700). There was a U-shaped relationship between dietary magnesium and RA in women, and our study highlights the importance of moderate dietary magnesium intake in possibly exerting a protective role in women with RA. © Author(s) (or their employer(s)) 2020. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.