Adolescent birth rates in Lao People's Democratic Republic (PDR) remain the highest in Southeast Asia. There is growing recognition that adolescent pregnancy in Lao PDR is occurring within and outside marriage, but there is a lack of robust qualitative evidence to understand girls' pathways to adolescent pregnancy and contributing factors, especially outside of union (cohabitation or marriage). This study aimed to improve understanding of pathways to adolescent pregnancy in Lao PDR among girls who experienced pregnancy at age 18 or below. We conducted participatory timeline interviews with 57 girls from urban, peri-urban, and rural communities in Vientiane Capital, Vientiane Province, and Luang Namtha, and follow-up interviews with a subset of 20 girls. We identified six pathways to pregnancy, including pathways outside (n = 23) and within union (n = 34). Outside-union pathways diverged according to the nature of sex preceding pregnancy (consensual/pressured, or forced), and pregnancy intention (unplanned, partner-led, or planned). Within-union pathways diverged according to the nature of the relationship before union (romantic or no romantic relationship/arranged union), who initiated the union (couple/girl, parent/partner, or pressured), and pregnancy intention. Factors contributing to girls' pregnancy included barriers to sexual and reproductive health (SRH) information and services; partner's control over reproductive decision-making; male sexual entitlement and alcohol use driving pressured/forced sex; cultural acceptance of child marriage and early union; and attitudes and norms regarding sex and pregnancy outside of union. Our findings support strengthening comprehensive sexuality education, including a focus on addressing myths about contraception, building girls' and boys' communication skills, engaging in respectful relationships, and addressing harmful gender norms. Our findings also highlight the need to improve girls' access to adolescent-responsive SRH services, address harmful substance use, challenge sociocultural barriers to young people accessing SRH information and services, and respond to sociocultural and financial drivers of child marriage/early union that contribute to adolescent pregnancy.