Background <p>Preterm birth is a leading cause of neonatal death and long-term health issues. Both environmental and social stressors, such as heat exposure and housing stability, may contribute to adverse birth outcomes, yet their joint effects are not well studied, especially in the Korean context. Therefore, we aimed to investigate the independent and joint associations of ambient temperature and housing instability with preterm birth.</p> Methods <p>This study analyzed eight years (2015–2022) of birth outcomes among women, derived from nationwide, register-based census data covering 100% of the population. The data included a cohort of individuals born in South Korea between 1983 and 1995 and their offspring. Logistic regression was performed using the average temperature during the month prior to delivery, housing instability, individual- and district-level covariates, and year and month.</p> Results <p>Among 1,579,033 singleton births, 79,399 (5.0%) were preterm. A higher ambient temperature during the month before delivery was significantly associated with increased risk of preterm birth (odds ratio for each 1ºC increase: 1.002 (95% confidence interval: 1.002, 1.003)). Housing instability, defined by poor housing quality, was also independently associated with a higher risk of preterm birth (odds ratio: 1.196 (95% confidence interval: 1.054, 1.358)). Both overcrowding and aging buildings also showed an increase in risk of preterm birth (odds ratio of 1.080 (1.019, 1.145) and 1.040 (1.013, 1.068), respectively). However, no significant interaction between heat exposure and unstable housing conditions was observed (odds ratio: 0.995 (95% confidence interval: 0.987, 1.003)).</p> Conclusions <p>The results highlight the compounded risks from both environmental and social stressors, highlighting the need for integrated public interventions that target both health and housing issues to help lower preterm birth rates.</p>