The development of prosocial skills in children is a key predictor of long-term social, cognitive, and emotional functioning. However, the role of fathers' psychological characteristics in fostering prosocial development, including during the prenatal period, and the mechanisms underlying their influence, remain relatively unexplored. This study aimed to examine whether a higher tendency of alexithymia, a difficulty to identify and verbalize emotions, in expectant fathers predicts prosocial behavior of two-year-old toddlers through the quality of coparenting and whether greater testosterone increase during a stressful parenting task moderates this indirect effect. A sample of 105 couples and their children was tracked longitudinally starting from the third trimester of pregnancy (T1), at three months (T2), and at two years postnatally (T3). Using self-report questionnaires, fathers reported on alexithymia (T1) and mothers and fathers reported on coparenting quality (T2). Additionally, fathers provided saliva samples before and after engaging in a stressful parenting task (the Inconsolable Doll Task) to measure testosterone reactivity (T1). Children's prosocial behavior was observed during an out-of-reach task (T3). A moderated mediation analysis using structural equation modeling showed that higher levels of alexithymia pre-birth predicted lower coparenting quality three months after birth, which in turn predicted lower prosocial behavior of two-year-old children, but only among fathers with mean or high testosterone increases. This study illuminates a potential mechanism by which fathers' alexithymia and testosterone reactivity forecast their toddlers' prosocial behavior. •Poorer coparenting predicts fewer expressions of prosocial behaviors in toddlers.•Greater prenatal alexithymia predicts prosocial behavior through co-parenting.•Testosterone increase moderates the indirect effect of alexithymia on prosociality.•Parental bio-psycho-social processes forecast the toddler's prosocial behaviors.