Communication and language barriers isolate Deaf American Sign Language (ASL) users from mass media, health care messages, and health care communication, which, when coupled with social marginalization, places them at a high risk for inadequate health literacy. Our objectives were to translate, adapt, and develop an accessible health literacy instrument in ASL and to assess the prevalence and correlates of inadequate health literacy among Deaf ASL users and hearing English speakers using a cross-sectional design. A total of 405 participants (166 Deaf and 239 hearing) were enrolled in the study. The Newest Vital Sign was adapted, translated, and developed into an ASL version (ASL-NVS). We found that 48% of Deaf participants had inadequate health literacy, and Deaf individuals were 6.9 times more likely than hearing participants to have inadequate health literacy. The new ASL-NVS, available on a self-administered computer platform, demonstrated good correlation with reading literacy. The prevalence of Deaf ASL users with inadequate health literacy is substantial, warranting further interventions and research.