The 2021 Expanded Child Tax Credit (ECTC) provided families with children monthly payments from July 2021 to December 2021. The association of this policy with adult health is understudied. To examine changes in adult self-reported health and household food security before and during ECTC monthly payments. This repeated cross-sectional study used multivariable regression with a difference-in-differences estimator to assess adult health and food security for 39 479 respondents to the National Health Interview Survey (January 2019 to December 2021) before vs during monthly payments. Analyses were stratified by income to focus on low-income vs middle-income and upper-income households. Eligibility for ECTC monthly payments from July 2021 to December 2021. Overall self-reported adult health and household food security as binary outcomes (excellent or very good health vs good, fair, or poor health; food secure vs food insecure). In this nationally representative cross-sectional study of 39 479 US adults (mean [SD] age, 41.0 [13.0] years; 7234 [21.7%] Hispanic, 321 [0.9%] non-Hispanic American Indian/Alaska Native, 2205 [5.7%] non-Hispanic Asian, 5113 [13.7%] non-Hispanic Black, and 23 704 [55.8%] White individuals), respondents were predominantly female (21 511 [52.4%]), employed (33 035 [86.7%]), and married (19 838 [55.7%]). Before disbursement of ECTC monthly payments, 7633 ECTC-eligible adults (60.1%) reported excellent or very good health, and 10 950 (87.8%) reported having food security. Among ECTC-ineligible adults, 10 778 (54.9%) reported excellent or very good health and 17 839 (89.1%) reported food security. Following disbursement of monthly payments, ECTC-eligible adults experienced a 3.0 percentage point (pp) greater adjusted increase (95% CI, 0.2-5.7) in the probability of reporting excellent or very good health compared with ECTC-ineligible adults. Additionally, ECTC-eligible adults experienced a 1.9 pp greater adjusted increase (95% CI, 0.1-3.7) in the probability of food security than ECTC-ineligible adults. In income-stratified analyses, the association between ECTC eligibility and overall health was concentrated among middle-income and upper-income households (3.7-pp increase in excellent or very good health; 95% CI, 0.5-6.9). Conversely, the association between ECTC eligibility and food security was concentrated among low-income adults (3.9-pp increase in food security; 95% CI, 0-7.9). The results of this cross-sectional study suggest that monthly ECTC payments were associated with improved adult overall health and food security. Cash transfer programs may be effective tools in improving adult health and household nutrition.