Young men who have sex with men (YMSM) are at increased risk for HIV and STI infection. While encouraging HIV and STI testing among YMSM remains a public health priority, we know little about the cultural competency of providers offering HIV/STI tests to YMSM in public clinics. As part of a larger intervention study, we employed a mystery shopper methodology to evaluate the LGBT cultural competency and quality of services offered in HIV and STI testing sites in Southeast Michigan (n = 43).We trained and deployed mystery shoppers (n = 5) to evaluate the HIV and STI testing sites by undergoing routine HIV/STI testing. Two shoppers visited each site, recording their experiences using a checklist that assessed 13 domains, including the clinic's structural characteristics and interactions with testing providers. We used the site scores to examine the checklist's psychometric properties and tested whether site evaluations differed between sites only offering HIV testing (n = 14) versus those offering comprehensive HIV/STI testing (n = 29). On average, site scores were positive across domains. In bivariate comparisons by type of testing site, HIV testing sites were more likely than comprehensive HIV/STI testing clinics to ascertain experiences of intimate partner violence, offer action steps to achieve safer sex goals, and provide safer sex education. The developed checklist may be used as a quality assurance indicator to measure HIV/STI testing sites' performance when working with YMSM. Our findings also underscore the need to bolster providers' provision of safer sex education and behavioral counseling within comprehensive HIV/STI testing sites.