--- - |2+ Abstract The placenta adapts to maternal environment and its alterations may have a lasting impact on child's temperament development. Prenatal stress has been linked to both a downregulation of monoamine oxidase A (MAOA) gene expression in the placenta and to difficult temperament. Capitalizing on an ongoing longitudinal study, we analysed data from 95 mother–child dyads to investigate whether MAOA mediates the association between prenatal stress and infant temperament. Prenatal stress was defined as exposure to Superstorm Sandy (Sandy) during pregnancy. Infant temperament was measured by Infant Behaviour Questionnaire‐Revised. MAOA gene expression was quantified in placenta tissue. The Smiling and Laughter subscale score was independently associated with Sandy exposure and MAOA placental gene expression. Mediation analysis confirmed that MAOA expression partially mediated the relationship between Sandy and Smiling and Laughter subscale, suggesting that in utero exposure to Sandy could induce lower frequency of smiling and laughter via downregulation of placental MAOA gene expression. These effects could compromise optimal temperamental trajectory and contribute to risk for psychological problems. Placental epigenetic markers can contribute to a multidimensional model of early intervention for high‐risk children. Highlights The study examined whether placental MAOA gene expression operates as a biological mediator in the relationship between prenatal stress and infant temperament. Mediation analyses revealed a partial mediation between prenatal stress and Smiling and Laughter subscale. Stress in utero could change MAOA gene expression epigenetically, which could shape temperament development. Such placental gene expressions may become useful biomarkers for screening and intervention. - Infant and Child Development, Volume 27, Issue 4, July/August 2018.