Many women report postpartum perineal pain due to perineal trauma after vaginal delivery. Perineal pain after giving birth declines over time; however, perineal trauma and pain negatively impact on the women's quality of life and their ability to care for their children. The degree of perineal trauma and instrument delivery with episiotomy are associated with perineal pain. Nevertheless, no studies have examined factors related to postpartum perineal pain, including weight changes during pregnancy as well as the course of delivery so far. We aimed to elucidate obstetric factors associated with perineal pain after vaginal delivery on the first postnatal day in Japanese primiparous women. A cross-sectional study conducted in five maternity hospitals in Japan included 142 primiparous women who vaginally delivered full-term and singleton infants. Perineal pain on the first postnatal day was evaluated using a visual analog scale. The final analysis included 92 participants with a mean age of 30.3 4.6 years. The median visual analog scale score was 54.0 mm. Multiple linear regression analysis demonstrated that gestational weight gain above the recommended Japanese range was positively and significantly associated with perineal pain on the first postnatal day, independent of maternal age, episiotomy, painkiller use, and neonatal birth weight. This finding may provide additional evidence regarding gestational weight gain within the recommended range based on the pre-pregnancy body mass index to reduce perineal pain on the first postnatal day.