The prevalence of vaccination against hepatitis B virus (HBV), factors associated with vaccination, and missed opportunities for vaccination were assessed among 949 street-recruited young injecting heroin users (IHUs) and noninjecting HUs (NIHUs). A cross-sectional study was carried out in Madrid, Barcelona, and Seville. Face-to-face interviews were held using a structured questionnaire with computer-assisted personal interviewing. Dried blood spot samples were tested for anti-HBV core antigen and HBV surface antigen. Bivariate and logistic regression analyses were performed. The prevalence of HBV vaccination was 21.7%, with significant differences among the cities (13.3% in Madrid, 18.4% in Seville, and 33.2% in Barcelona) and between IHUs (23.8%) and NIHUs (17.9%). In the logistic regression analysis, living in Barcelona and being aged 25 years or younger were associated with HBV vaccination in IHUs and NIHUs; in IHUs, vaccination was also associated with living in the street or in institutions for most of the last 12 months. Practically all those susceptible to HBV infection had missed at least one opportunity for vaccination, and most of them had missed such an opportunity in the last year. The proportion of vaccinated HUs remains very low despite efforts to set up harm reduction programs. New and more active strategies must be incorporated in these programs.