Pregnant women who smoke are at a high risk of preterm birth (PTB) and have low partner-support. In a prospective cohort study, we aimed to examine the role of partner-support in gestational duration and PTB among pregnant women who smoke as well as the interaction with race/ethnicity. We analyzed secondary data of 53 participants from the University at Buffalo Pregnancy and Smoking Cessation Study. Partner-support was measured with Turner's support scale where women reported how much they agreed with five statements about how supportive their partner was. Total partner-support was calculated and split into emotional support and accountability. We fit multivariable linear regression models and log-binomial regression models for gestational duration and PTB, respectively. Gestational duration significantly increased with partner-support (0.22weeks longer per unit increment in partner-support score), emotional support (0.52weeks), and accountability (0.35weeks). This association tended to be stronger among Hispanics and women of other races than non-Hispanic Caucasians and African Americans. Women with a bed partner had 1.48weeks longer gestational duration than women without a bed partner. Partner-support may increase gestational duration and reduce PTB risk among pregnant women who smoke, especially among Hispanic women. Sharing a bed with a partner was associated with a longer gestational duration. Our findings may be interpreted with caution due to limitations such as small sample size, recruitment within a single metropolitan area, and partner-support measurement via maternal reports only. A partner-support intervention to increase gestational duration is warranted.