Introduction Muscle weakness is an important indicator of disability, chronic disease, and early mortality. Grip strength is a simple, cost-effective measure of overall muscle strength. The Foundation of the National Institutes of Health recently proposed sex-specific grip strength cut points for clinical muscle weakness. However, these criteria were established using non−nationally representative data. This study used nationally representative data on Americans aged ≥65 years to identify race- and sex-specific cut points for clinical muscle weakness and quantify prevalence among older blacks and whites by sex. Methods Classification and Regression Tree models were used to identify cut points based on individual-level grip strength associated with slow gait speed (<0.8 m/second) among 7,688 individuals (57% female; 8% black; mean age, 74.6 [SD=6.79] years) from the 2010/2012 Health and Retirement Study during January−April 2016. Identified cut points were then used to quantify the prevalence of weakness by race/sex subgroup. Results Fifty-five percent of men (maximum grip strength <39 kg) and 47% of women (maximum grip strength <22 kg) were classified as weak. Higher cut points were identified for black men (maximum grip strength <40 kg) and women (maximum grip strength <31 kg), and the prevalence of weakness (57% and 88%, respectively) was higher compared with whites. Fifty-five percent of individuals had slow gait speed (<0.8 m/second). Conclusions Prevalence of weakness was substantially higher than previous reports, underscoring the importance of using population-level data to identify individuals at greatest risk for adverse health outcomes. This is the first study to establish cut points for muscle weakness in a nationally representative sample by race and sex.