Aim Unintended pregnancy is a significant public health concern in South Asian countries because of its negative association with the socioeconomic and health outcomes for both children and mothers. The present study aimed to explore the prevalence of unintended pregnancy and explore its determinants among women of reproductive age in six South Asian countries. Methods Nationwide latest demography and health survey data from six South Asian countries, including Bangladesh (2014), Pakistan (2017-2018), Nepal (2016), Afghanistan (2015), Maldives (2016-2017) and India (2015-2016) were pooled for the present study. Multivariate analysis was performed to explore the association between unintended pregnancy and its associated factors. Results Amongst the total women (n = 41,689), overall, 19.1% pregnancies were reported as unintended (ranging from 11.9% in India to 28.4% in Bangladesh). The logistic regression model showed that younger women (15-19 years) had 1.42 times higher chance of unintended pregnancies. The odds of unintended pregnancies was 1.24 times higher for poorest women and 1.19 times higher for poorer women. Further, urban women (aOR = 0.70, 95% CI = 0.50-0.80), women having no children (aOR = 0.10, 95% CI = 0.09-0.12), smaller ([less than or equal to]4) family (aOR = 0.72, 95% CI = 0.67-0.78), those who intent to use contraceptive (aOR = 0.72, 95% CI = 0.60-0.86), currently living with partner (aOR = 0.90, 95% CI = 0.81-0.99), first cohabitation in teenage ([less than or equal to]19 years) (aOR = 0.85, 95% CI = 0.78-0.92) were less likely to report unintended pregnancies. Conclusions This study has showed that women's age, wealth index, place of residence, number of children, family size, the intention of contraceptive use, living with a partner, and first cohabitation age are essential determinants of unintended pregnancy. These factors should be considered when trying to reduce unintended pregnancy in six South Asian countries. However, there is a need to improve health education, counselling, skills-building, sex education, modern contraceptive use and its access in this region. Intervention programs regarding reproductive health and policies are warranted to reduce rates of unintended pregnancy in South Asian countries.