Objective To investigate the effect of antenatal corticosteroids (ACS) on the risk of transient tachypnea of the newborn (TTN) and respiratory distress syndrome (RDS) within 24 hours after birth in late preterm infants born to mothers with gestational diabetes mellitus (GDM). Methods Clinical data of mothers with GDM and their late preterm infants admitted to the Department of Obstetrics, Xiamen Maternal and Child Health Hospital, from January 2017 to December 2023 were retrospectively reviewed. Based on whether mechanical ventilation was required within 24 hours after birth, infants were classified into a mechanical ventilation group (n=322) and a control group (n=1 098), and perinatal and maternal characteristics were compared. According to the interval from the first ACS dose to delivery, infants were categorized into <2 days (n=399), 2-7 days (n=305), and >7 days (n=60) groups; according to ACS dosage, they were categorized into no ACS (n=656), incomplete course (<2 doses; n=399), and complete course (≥2 doses; n=365) groups. Associations between ACS timing/dose and TTN and RDS were analyzed. Results A total of 1 420 infants were included. Multivariable logistic regression showed that ACS administration was a protective factor against the need for mechanical ventilation within 24 hours after birth (OR=0.125, 95%CI: 0.085-0.183). A complete ACS course was associated with a more pronounced reduction in the mechanical ventilation rate (OR=0.080, 95%CI: 0.049-0.130) and a lower incidence of TTN (P<0.001), while the incidence of RDS did not differ significantly (P>0.05). An interval of >7 days from the first ACS dose to delivery had the most marked association with reduced postnatal mechanical ventilation (OR=0.127, 95%CI: 0.047-0.348). Conclusions ACS does not reduce the incidence of RDS in late preterm infants of mothers with GDM, but it effectively reduces TTN and the need for mechanical ventilation within 24 hours after birth. A complete ACS course and an interval of >7 days from the first dose to delivery provide the greatest benefit in reducing TTN and early postnatal mechanical ventilation.