IMPORTANCE: Substance use disorders (SUDs) are major contributors to morbidity and mortality globally, but they are often underrecognized and underdiagnosed, particularly in some sociodemographic subgroups. Understanding the extent to which clinical diagnoses underestimate these conditions within subgroups is imperative to achieving equitable treatment, regardless of race, ethnicity, gender, or age, and to informing and improving performance monitoring. OBJECTIVE: To compare clinically documented diagnosis rates of alcohol use disorder (AUD), drug use disorder (DUD), and total SUD (AUD and/or DUD) with the prevalence of these disorders as reported in surveys-based on structured, validated diagnostic assessments-across demographic subgroups. DESIGN, SETTING, AND PARTICIPANTS: A telephone-based survey was conducted from January 8, 2018, to April 30, 2019, among 5995 Veterans Health Administration (VHA) outpatients who were randomly sampled from 30 VHA facilities and were 18 years of age or older, could complete the survey in English, and had a valid address and telephone number. Survey data were linked to electronic health record (EHR) data for all participants. Statistical analysis was performed between January 29, 2020, and April 20, 2021. EXPOSURES: Demographic subgroups based on self-report: gender (male or female), age (18-34, 35-49, 50-64, 65-74, and ≥75 years), and race and ethnicity (Black non-Hispanic, Hispanic, multiracial, other [Asian or Asian-American, American Indian or Alaskan Native, Native Hawaiian or Pacific Islander, and any other race endorsed by the participant], and White non-Hispanic). MAIN OUTCOMES AND MEASURES: Survey-based prevalence rates of AUD, DUD, and SUD were assessed using the Mini International Neuropsychiatric Interview, version 7.0, the only validated instrument available at study outset that measured Diagnostic and Statistical Manual of Mental Disorders (Fifth Edition) criteria for past 12-month diagnoses. Clinically documented diagnosis rates were measured using International Statistical Classification of Diseases and Related Health Problems, Tenth Revision diagnoses from VHA EHR data. Analyses compared survey-based prevalence rates of AUD, DUD, and SUD with diagnosis rates using sensitivity and specificity and difference-in-difference analysis. All analyses were weighted with survey weights to account for nonresponse. RESULTS: Of 5995 participants, 4115 (68.6%) were White non-Hispanic, and 5429 (91.1%) were male; the mean (SD) age was 61.5 (15.3) years. The survey-based prevalence rates of AUD, DUD, and SUD were higher than the diagnosis rates among all patients (AUD, 608 [10.1%] vs 360 [6.0%]; DUD, 282 [4.7%] vs 275 [4.6%]; SUD, 768 [12.8%] vs 515 [8.6%]). Survey-based prevalence rates of AUD and SUD exceeded the diagnosis rates in every demographic subgroup. Gaps between diagnosis rates and survey-based prevalence rates for AUD and SUD were largest among patients aged 18 to 34 years (AUD diagnosis rate, 27 [6.9%; 95% CI, 4.8%-9.9%] vs AUD prevalence rate, 88 [22.4%; 95% CI, 17.3%-28.5%]; SUD diagnosis rate, 41 [10.5%; 95% CI, 8.1%-13.4%] vs SUD prevalence rate, 109 [27.7%; 95% CI, 22.6%-33.3%]) and Hispanic and Latinx patients (AUD diagnosis rate, 31 [7.6%; 95% CI, 5.3%-10.8%] vs AUD prevalence rate, 72 [17.7%; 95% CI, 14.0%-22.1%]; and SUD diagnosis rate, 48 [11.7%; 95% CI, 7.9%-16.9%] vs SUD prevalence rate, 88 [21.6%; 95% CI, 18.0%-25.8%]). For DUD, only patients aged 18 to 34 years had a true prevalence rate that significantly exceeded the diagnosis rate (diagnosis rate, 21 [5.4%; 95% CI, 3.7%-7.8%] vs prevalence rate, 40 [10.1%; 95% CI, 7.2%-14.0%]). CONCLUSIONS AND RELEVANCE: The results of this survey study suggest that existing diagnostic procedures and tools are insufficient to capture SUD prevalence rates, particularly among younger patients and Hispanic and Latinx patients. Clinics and health systems should implement standardized SUD assessments to ensure the provision of equitable care and the optimal identification of underlying conditions for performance monitoring.