Vaccination against pertussis during pregnancy is recommended to protect newborns, yet there is limited information about the effectiveness of maternal tetanus toxoid, reduced diphtheria toxoid, acellular pertussis (Tdap) vaccine before the first infant dose of diphtheria, tetanus and acellular pertussis (DTaP) vaccine and during the first year of life in infants who have received DTaP. In a retrospective cohort study of infants born at Kaiser Permanente Northern California from 2010 to 2015, we estimated the effectiveness of maternal pertussis vaccination for protecting newborns against pertussis in the first 2 months of life and in the first year of life accounting for each infant DTaP dose. Among 148 981 newborns, the vaccine effectiveness of maternal Tdap was 91.4% (95% confidence interval [CI], 19.5 to 99.1) during the first 2 months of life and 69.0% (95% CI, 43.6 to 82.9) during the entire first year of life. The vaccine effectiveness was 87.9% (95% CI, 41.4 to 97.5) before infants had any DTaP vaccine doses, 81.4% (95% CI, 42.5 to 94.0) between doses 1 and 2, 6.4% (95% CI, -165.1 to 66.9) between doses 2 and 3, and 65.9% (95% CI, 4.5 to 87.8) after infants had 3 DTaP doses. Maternal Tdap vaccination was highly protective against infant pertussis, especially in the first 2 months of life. Even after infant DTaP dosing, there was evidence of additional protection from maternal Tdap vaccination for the first year of life. This study strongly supports the United States’ current recommendation to administer Tdap during each pregnancy.