Inconsistent results have been obtained from studies investigating the association between soil-transmitted helminths (STHs) and nutritional status and cognitive outcomes. This study aimed to investigate the influence of STHs and nutritional status on the intelligence quotient (IQ) of adolescents in the Chitwan Valley, Nepal. In this birth cohort study from the Chitwan district of Nepal, 74 cohort participants were followed up after 14 years. The presence and egg density of the STHs were investigated following the Kato-Katz method. Full-scale IQ was evaluated using the Wechsler Abbreviated Scale of Intelligence-II (WASI-II). Multivariate regression analysis was also conducted to examine the associations of STHs and nutritional status with IQ scores, adjusting for adverse childhood experiences (ACE) and demographic and socioeconomic factors. The only STHs detected was for Ascaris lumbricoides, which was detected in 16% (12/74) of the study participants. Similarly, 16% (12/73) of participants reported undernutrition (body mass index (BMI) Z score<-2SD), and 33% (25/74) of participants had a developmental deficit (WASI II FSIQ < 69). There were no significant associations between the presence of A. lumbricoides or egg density and IQ scores. However, undernutrition (BMIZ<-2SD) increased the odds of a developmental deficit according to both univariate and adjusted multivariate models [i.e., adjusted odds ratio (aOR) 9.41; 95% CI (2.23 to 39.80)]. While the study primarily examines the association of STHs and nutritional status with IQ, it is important to note that adolescents with educated mothers or higher incomes also showed better cognitive development. The prevalence of undernutrition and associated developmental deficits characterized by an IQ < 69 was alarmingly high in this cohort of adolescents from Chitwan, Nepal. Despite the high prevalence of A. lumbricoides, its infection was not identified as significantly associated with cognition. However, it is noteworthy that a consistent favorable impact of an enriching postnatal environment, approximated by maternal education and family income, may improve neurodevelopmental indicators.