Objective <p>The purpose of this study was to examine the association of folate levels, including red blood cell (RBC) and serum folate with mortality (cardiovascular disease (CVD)–related, all-cause, and cancer-related) in patients with arthritis.</p> Methods <p>We integrated and analyzed the data from the 1999–2018 National Health and Nutrition Examination Survey to conduct this study. Weighted Cox proportional hazard regression, restricted cubic spline (RCS) model, and subgroup analysis were used to analyze the association of RBC and serum folate levels with all-cause, cancer-related, and CVD-related mortality. Additionally, according to the folate levels quartiles, the differences in survival rate of RBC and serum folate with all-cause, cancer-related, and CVD-related mortality were showed in the Kaplan–Meier survival curves.</p> Results <p>Our analysis included 12,332 individuals in total. The RCS showed the U-curve association of RBC and serum folate with CVD-related, all-cause, and cancer-related mortality in patients with arthritis. In addition, patients with arthritis in the highest quartile group of RBC and serum folate had the highest risk of CVD-related and all-cause mortality (all Log-rank <i>P</i> &lt; 0.001).</p> Conclusions <p>RBC and serum folate concentrations are associated with U-shaped mortality (all-cause and CVD-related) in patients with arthritis in American, and maintaining an appropriate range of serum folate and RBC folate may promote public health.<Table Float="No" ID="Taba"> <tgroup cols="2"> <colspec align="left" colname="c1" colnum="1" /> <colspec align="left" colname="c2" colnum="2" /> <tbody> <row> <entry nameend="c2" namest="c1"> <p><b>Key Points</b></p> <p>• <i>Folate levels have U-shaped association with risk of mortality in patients with arthritis.</i></p> <p>• <i>The potential mechanisms of folate levels in mortality of patients with arthritis need to be further explored.</i></p> </entry> </row> </tbody> </tgroup> </Table></p>