Sedentary time (ST) has been associated with frailty, but whether this association differs by highintensity recreational physical activity (PA) remains unclear. This study examined the association between ST and frailty and the modifying role of high-intensity recreational PA among U.S. adults. A total of 19,656 adults aged ≥ 20 years were included. Frailty was assessed via a validated 49-item Frailty Index. ST was categorized as < 4, 4–6, 6–8, and ≥ 8 hours/day. High-intensity recreational PA was defined as any self-reported vigorous recreational PA recorded by NHANES, based on activities performed continuously for at least 10 minutes. Weighted logistic regression and restricted cubic spline (RCS) models were used to account for the complex survey design, with adjustments for key covariates. The interaction between ST and high-intensity recreational PA was also evaluated. Among participants reporting high-intensity recreational PA, lower odds of frailty were observed in the 4–6 and 6–8 h/day ST groups compared with the < 4 h/day group (adjusted OR = 0.49; 95% CI: 0.27–0.88 and 0.49, 0.26–0.93, respectively; P < 0.05). In contrast, among those without high-intensity recreational PA, prolonged ST was associated with higher odds of frailty. RCS indicated a linear association between ST and frailty overall, with non-linear patterns observed when stratified by PA status. High-intensity recreational PA modified the observed association between ST and frailty among U.S. adults. These stratified findings should be interpreted cautiously because of the cross-sectional design and potential residual confounding.