Understanding HIV-related behaviours and the factors that influence these behaviours among people living with HIV (PLHIV) is critical to the design of effective HIV-prevention strategies; however, this subject has yet to receive the attention it deserves in Vietnam. Given that greater proportions of new HIV infections in the country stem from heterosexual transmission, it is essential to examine the sexual behaviours of Vietnamese PLHIV. The purpose of this qualitative study was to explore the sexual behaviour of individuals following HIV diagnosis and to gain insight into how and why HIV diagnosis affects sexual practices and relationships. Seventy PLHIV in Thaibinh province participated in semi-structured, in-depth interviews. Qualitative data were supported by a quantitative questionnaire on demographics and sexual and drug use history. Nearly all of the participants reported adopting safer sexual practices following HIV diagnosis by using condoms consistently and reducing the number of sex partners. This was true for injecting drug users, female sex workers, unmarried individuals and participants in both HIV serodiscordant and seroconcordant marriages. Motivations for adopting these preventive measures included avoiding HIV transmission, reinfection or cross-resistance as well as preservation of one's own health. Due to stigma, depression, fear of transmission, health status and/or drug addiction, HIV diagnosis dramatically impacted the sexual health of most participants by reducing sexual desire, pleasure and frequency. Implications for HIV prevention and care programmes and policies in Vietnam are discussed.