The peripartum period is a time of increased vulnerability to stress for women, yet no consensus definition of maternal peripartum stress exists. This paper reports work from European COST Action 22114 (TREASURE), which identified key concepts related to maternal peripartum stress in the literature and developed a consensus definition via a qualitative Delphi process involving 161 international researchers and clinicians. The resulting definition supports a shared understanding, enabling more targeted research and evidence-based clinical practice to improve perinatal care.