Background: To estimate the proportion of adults’ and parents’ knowledge of the adult aerobic and youth physical activity guidelines, respectively, in the United States. Methods: Data were analyzed from a national sample of adults in the 2017 ConsumerStyles survey. Prevalence of knowledge of the adult aerobic guideline (ie, 150 min/wk of moderate-intensity activity) was estimated among all respondents (n = 3910) and of the youth guideline (ie, 60 min/d of physical activity on 7 d/wk) among parents (n = 1288). Odds ratios were estimated using logistic regression models adjusting for demographic characteristics. Results: Overall, 2.5% (95% confidence interval, 2.0–3.1) of adults and 23.0% (95% confidence interval, 20.5–25.7) of parents were knowledgeable of the adult aerobic and youth guidelines, respectively. After adjustment, odds of knowledge of the adult guideline differed significantly by sex and physical activity level, whereas knowledge of the youth guideline differed by parental education level. Conclusions: Despite the release of the 2008 Physical Activity Guidelines for Americans nearly a decade ago, most US adults and parents lack knowledge of the adult aerobic and youth physical activity guidelines. Effective communication strategies may help raise awareness of current and future editions of national guidelines for physical activity.