In recent decades, music therapy in the Neonatal Intensive Care Unit (NICU) has been shown to regulate preterm infant's physiological responses and improve maternal mental health. This study investigated the effects of the music therapy intervention for the mother–preterm infant dyad (MUSIP) for maternal anxiety, postnatal depression, and stress, and preterm infants’ weight gain, length of hospitalization, heart rate (HR), and oxygen saturation (So2). A pre‐experimental design was used with 33 mother–preterm infant dyads in a Brazilian NICU: 16 dyads in the Music Therapy Group (MTG) and 17 dyads in the Control Group (CG). The MTG took part in the MUSIP, aimed at supporting maternal singing with the preterm baby. Infants’ HR and So2 were recorded at each minute from 10 min before to 10 min after sessions 1, 3, and 6. Before infants’ discharge, maternal anxiety and depression scores were lower in the MTG compared to the CG. Anxiety, depression, and stress levels decreased significantly after the intervention in the MTG. With regard to infants, HR and So2 ranges were higher during music therapy, compared to before and after sessions. MUSIP improved maternal mental health and affected preterm infants’ emotional arousal, with positive trends in decreasing HR, stabilizing So2, and reducing length of hospitalization.