Objectives: Both maternal body mass index (BMI) and smoking during pregnancy have been associated with a range of adverse maternal and infant birth outcomes. This study aimed to identify whether these independent variables had an interacting relationship with small for gestational age in an Australian obstetric cohort. Study design: A retrospective cohort design used data from the Birthing Outcomes System of a major tertiary hospital in Australia. Methods: A total of 14,487 singleton births between January 2008 and December 2013 were included in the analysis. Chi-squared tests and one-way analysis of variance were used for the comparison of categorical and continuous variables, respectively. Adjusted odds ratios (AORs) were calculated to determine the association of smoking status with the outcome variable of interest, and these are reported for each maternal BMI category. Results: Of the 14,487 women, 716 (4.9%) were underweight (BMI ≤18 kg/m2), 7268 (50.2%) had healthy weight (BMI = 19–24 kg/m2), 3658 (25.3%) were overweight (BMI = 25–29 kg/m2), 1558 (10.8%) had class I obesity (BMI = 30–34 kg/m2), 711 (4.9%) had class II obesity (BMI = 35–39 kg/m2) and 576 (3.9%) had class III obesity (BMI = 40+ kg/m2). Of all women, 10.8% reported being current smokers, 82.0% reported to have never smoked and 4.0% reported to have stopped smoking during or before pregnancy. Smokers with a BMI ≥40 kg/m2 were 4.5 (AOR = 4.508; 95% confidence interval: 2.068–9.828) times more likely to give birth to a small-for-gestational-age infant than non-smokers within the same BMI category. This increased risk was not observed in women who ceased smoking before or during pregnancy. Conclusions: Our study supports the efficacy of antismoking policies within maternal public health. In addition, greater support with respect to smoking cessation is indicated for women during pregnancy with an elevated BMI.