Despite growing attention to other social needs like food and housing insecurity, financial strain, defined as having difficulty making ends meet or lacking money for basic needs, is under-recognized. Inconsistent labels and measures have made the literature difficult to unify. We used many synonyms for financial strain to systematically identify 199 U.S. studies (316 papers) that used financial strain measures that were operationally consistent with our definition as predictors of health among adults. We thematically coded financial strain measures for content and synthesized evidence based on measure and methods. Financial strain was measured by self-reported lacking money for basic needs (119 studies) and/or difficulty making ends meet (n = 132), and less commonly additionally based on coping strategies (n = 23), satisfaction with finances (n = 14), worry about finances (n = 22), the anticipation of strain (n = 14), and/or lacking money for leisure (n = 29). Regardless of measure, financial strain was associated with poorer mental, physical, biological, and functional health, worse health behaviors and more social needs. Associations were found across diverse and population-based samples and when accounting for other socioeconomic factors and even intermediating health factors. Results demonstrate predictive validity for two different one-item screening tools. Furthermore, the vast evidence linking financial strain to health highlights an urgent need for policy action addressing financial strain to advance health equity.