The French Institute for Public Health Surveillance conducted two seroprevalence studies of the HBs antigen (HBsAg) in specific populations. The first, the ANRS-Coquelicot survey, examined drug users (DU) attending treatment and harm reduction centers between 2011 and 2013. The second, Prevagay, focused on men who have sex with men (MSM) frequenting gay social venues in Paris in 2009. Participants recruited for these two studies provided a blood sample via self-collection of capillary blood from their fingertips onto a blotter, then completed a behavioral questionnaire. This questionnaire was anonymous and administered face-to-face by interviewers for IDUs, and self-administered for MSM. HBsAg testing was performed in the laboratory using the Monolisa HBsAg ULTRA ELISA kit (Biorad®). Among the 647 IDU men and 876 MSM surveyed in Paris and included in the analysis, 15 IDUs and 12 MSM were HBsAg-positive, representing seroprevalence rates of 2.1% (95% CI [1.0–4.2]) and 1.4%, respectively. These two specific populations, which differ in terms of sociodemographic profiles but are both exposed to the risk of HBV transmission due to their drug use or sexual practices, had HBsAg seroprevalence rates similar to those of the general population.