More than 30% of perinatally HIV-infected children in Thailand are 12 years and older. As these youth become sexually active, there is a risk that they will transmit HIV to their partners. Data on the knowledge, attitudes, and practices (KAP) of HIV-infected youth in Thailand are limited. Therefore, we assessed the KAP of perinatally HIV-infected youth and youth reporting sexual risk behaviors receiving care at two tertiary care hospitals in Bangkok, Thailand and living in an orphanage in Lopburi, Thailand. From October 2010 to July 2011, 197 HIV-infected youth completed an audio computer-assisted self-interview to assess their KAP regarding antiretroviral (ARV) management, reproductive health, sexual risk behaviors, and sexually transmitted infections (STIs). A majority of youth in this study correctly answered questions about HIV transmission and prevention and the importance of taking ARVs regularly. More than half of the youth in this study demonstrated a lack of family planning, reproductive health, and STI knowledge. Girls had more appropriate attitudes toward safe sex and risk behaviors than boys. Although only 5% of the youth reported that they had engaged in sexual intercourse, about a third reported sexual risk behaviors (e.g., having or kissing boy/girlfriend or consuming an alcoholic beverage). We found low condom use and other family planning practices, increasing the risk of HIV and/or STI transmission to sexual partners. Additional resources are needed to improve reproductive health knowledge and reduce risk behavior among HIV-infected youth in Thailand.