The goal of this study was to analyze how worry about sexually-transmitted infections (STI) and HIV influences attitudes and self-efficacy towards condom use, HIV-related knowledge, HIV-perceived susceptibility and HIV-misconceptions in a multicultural sample in Spain. The sample was composed of 3,051 adolescents aged between 14 and 19 years old who lived in Spain. Of these, 67.7% were native Spaniards and the remaining 32.3% were Latin American immigrants. Results showed that worry about STI and HIV has a direct influence on condom use self-efficacy and HIV-knowledge and HIV-perceived susceptibility. Native Spanish adolescents showed higher positive attitudes towards condom use, greater HIV-knowledge and HIV-perceived susceptibility, and lower negative attitudes towards condoms use and HIV-misconceptions than Latin American adolescents. In the discussion, the importance of worry about STI and HIV is highlighted as a mediator variable that can predict risky sexual behavior and is related to cultural origin.