Importance: Inequalities in preterm infant mortality exist between population subgroups within the US. Objective: To characterise trends in preterm infant mortality by maternal race and socioeconomic status to assess how inequalities in preterm mortality rates have changed over time. Design, setting, and participants: This was a retrospective longitudinal descriptive study using the US National Centre for Health Statistics Birth Infant/Death Dataset of 12,256,303 preterm infant births over 26 years, between 1995 and 2020. Exposures: Maternal characteristics including race, smoking status, educational attainment, antenatal care, and insurance status were used as reported on an infants’ US birth certificate. Main outcomes and measures: Preterm infant mortality rate was calculated for each year between 1995-2020 for all subgroups, with a trend regression coefficient calculated to describe the rate of change in preterm mortality. Results: The average US preterm infant mortality rate (IMR) decreased from 33.71 (95% CI, 33.71 to 34.04) per 1000 preterm births per year between 1995-1997, to 23.32 (95% CI 23.05 to 23.58) between 2018-2020. Black Non-Hispanic infants were more likely to die following preterm birth than White Non-Hispanic and Hispanic infants however once born extremely prematurely Black and Hispanic infants had a narrow survival advantage. The rate of decrease in preterm IMR was higher in Black infants (-0.015) than in White (-0.013) and Hispanic infants (-0.010); however, the relative risk of preterm IMR among Black infants compared to White infants remained the same between 1995-1997 vs 2018-2020 (RR 1.40, 95% CI 1.38 to 1.44, vs 1.43, 95% CI 1.39 to 1.46). The rate of decrease in preterm IMR was higher in non-smokers compared to smokers (-0.015 vs -0.010), in those with high levels of education compared to those with intermediate or low (-0.016 vs – 0.010, -0.011), and in those who had received adequate antenatal care compared to those who did not (-0.014 vs -0.012, -0.013). Overtime, the relative risk of preterm mortality widened within each of these subgroups. Conclusions and relevance: Between 1995 and 2020, US preterm infant mortality has improved amongst all categories of prematurity. Inequalities in preterm infant mortality based on a mother’s race have remained constant whilst socioeconomic disparities have widened over time.