Background: – Preterm infants experience physiological instability and mothers often experience heightened anxiety in the neonatal intensive care unit (NICU), prompting growing interest in combining kangaroo mother care (KMC) with auditory interventions such as lullaby singing to enhance maternal–infant well-being. Purpose: – This study aimed to examine the added effect of maternal lullaby singing during KMC on maternal state anxiety and the physiological parameters of mothers and their preterm infants. Methods: – This randomized controlled trial was conducted with 61 mother–preterm infant dyads in the NICU of a state hospital in southern Türkiye. Participants were allocated to either the KMC-only group (n = 31) or the KMC + lullaby group (n = 30). Maternal state anxiety was assessed at baseline and after the intervention period. Maternal and infant physiological parameters were monitored for 3 consecutive days. The study was registered at ClinicalTrials.gov (NCT06964204). Results: – Maternal state anxiety decreased significantly over time in the KMC + lullaby group compared with the KMC-only group (P < .05). Among infants, time-dependent significant differences were observed in respiratory rate and oxygen saturation. For mothers, significant between-group and time effects were found in heart rate, respiratory rate, and oxygen saturation (P < .05). Implications for Research: – Given their low cost and feasibility, incorporating lullaby singing into routine KMC may enhance both psychological and physiological outcomes for mothers and preterm infants. Integration of this combined approach into NICU care protocols is recommended, alongside further research to validate these additive effects.