The relationship between neighborhood social dynamics and adolescent sexual behavior has not been well explored. We conducted a cross-sectional survey with 343 adolescents recruited from two health clinics in Baltimore. Multivariate logistic regression was utilized to assess the influence of perceived neighborhood social cohesion and collective monitoring of youth on condom use at last sex, controlling for family and individual factors. Condom use was significantly higher among participants who perceived their neighborhoods as high, 54.7%, versus low, 40.4%, in social cohesion. Neighborhood cohesion was significantly associated with condom use in multivariate analyses, as was parental communication, family structure, and gender. No association between perceived neighborhood collective monitoring of youth and condom use was found. We conclude that perceived neighborhood social cohesion is positively associated with condom use among adolescents vulnerable to HIV/STI and should be encouraged in the context of community-based prevention efforts.