Body size dissatisfaction has been documented as a risk factor for obesity, but little is known about the effect of body size dissatisfaction on excessive gestational weight gain. The objective of the study was to determine the association of pre-pregnancy body size dissatisfaction with excessive gestational weight gain in Iranian pregnant women. This case-control study compared pre-gravid body satisfaction status in 182 women with excessive gestational weight gain and 180 women who gained weight within the guidelines of the Institute of Medicine. All the participants of the study were 35-41 weeks gestational age and received prenatal care in Shahid Akbarabadi Hospital. The women were asked to think back to their pre-pregnant state and report their body size satisfaction status measured by the Body Image Assessment for Obesity (BIA-O). According to this measurement, the women were divided into three categories: dissatisfied women with a thinner body size preference, dissatisfied women with a heavier body size preference and satisfied women. Among women with excessive gestational weight gain, 56.6% preferred a thinner body size, while 53.9% of those with adequate gestational weight gain were satisfied with their pre-gravid body sizes. After adjusting for cofounders, those with a thinner body size preference were more likely to gain weight excessively during pregnancy when compared to satisfied women (OR: 2.17, 95% CI: 1.17-4.02). Our result showed that thinner body size preference was associated with excessive gestational weight gain. Further studies are needed to investigate whether changes in women's feelings about their body sizes will decrease the proportion of women with excessive gestational weight gain.