The present study examines the relationship between sex, infant temperament, and childhood psychophysiological reactivity via electrodermal activity (EDA). Both temperament and EDA are known to be relatively stable traits across the lifespan reflecting individual reactivity and regulation linked to suboptimal behavioral development and risk for psychopathology. However, little is known about the role of sex in the relationship between temperament and EDA. As a part of a larger longitudinal study of behavioral development, 125 participants were followed from birth until the age of 3 years to examine the relationship between temperament and psychophysiological reactivity in different sex groups. Measurements of temperament at age 6 months, and EDA, via skin conductance response (SCR) rate to a series of six startling auditory stimuli at 3 years of age were collected. Median splits of SCR rate and three temperament dimensions (positive affect, negative affect, and regulation) were created to designate high/low groups. Results indicate sex moderated the relationships between temperament traits and SCR rates. Specifically, low positive affect was associated with an increased risk for high psychophysiological reactivity in boys (odds ratio = 3.8), whereas high regulation was associated with an increased risk for greater reactivity in girls (odds ratio = 4.2). While preliminary, these findings suggest the importance of sex in relation to psychophysiological and temperamental reactivity, risk factors for developmental psychopathology. As our participants age, follow-up research to investigate the stability of these associations will provide valuable insights for the potential of EDA as a psychophysiological marker for developmental psychopathology risk in young children.