BACKGROUND: Postpartum depression (PPD) affects up to one in five and negatively affects mothers, birthing parents, and their infants. The impact of PPD exposure on infant emotion regulation (ER) may be particularly harmful given its associations with later psychiatric problems. It remains unclear if treating maternal PPD can improve infant ER. OBJECTIVE: To examine the impact of a nine-week peer-delivered group cognitive behavioral therapy (CBT) intervention on infant ER assessed across physiological and behavioral levels. METHODS: Seventy-three mother-infant dyads were enrolled in a randomized controlled trial from 2018 to 2020. Mothers/birthing parents were randomized to the experimental group or waitlist control group. Measures of infant ER were collected at baseline (T1) and nine weeks later (T2). Infant ER was assessed using two physiological measures (frontal alpha asymmetry (FAA) and High Frequency-Heart Rate Variability (HF-HRV)), and parental-report of infant temperament. RESULTS: Experimental group infants displayed more adaptive changes in both physiological markers of infant ER from T1 to T2 (FAA (F(1,56) = 4.16, p = .046) and HF-HRV (F(1,28.1) = 5.57, p = .03)) than those in the waitlist control group. Despite improvements in maternal PPD, no differences were noted in infant temperament from T1 to T2. LIMITATIONS: A limited sample size, potential lack of generalizability of our results to other populations, and an absence of long-term data collection. CONCLUSIONS: A scalable intervention designed for those with PPD may be capable of adaptively improving infant ER. Replication in larger samples is needed to determine if maternal treatment can help disrupt the transmission of psychiatric risk from mothers/birthing parents to their infants.