This study examined the contributions of infant temperament and marital relationship quality to the quality of the early coparenting relationship for couples parenting 3.5-month-old infants. Marital quality was assessed observationally during the third trimester of pregnancy. When infants were 3.5 months old, infant temperamental characteristics (fussiness and unadaptability) were rated by parents and observers and coparenting behavior was assessed observationally in play and child care contexts. Results indicated that associations between infant temperament and coparenting behavior depended on marital quality: couples with high marital quality showed more optimal coparenting behavior when faced with a challenging infant, whereas couples with low marital quality showed less optimal coparenting behavior when caring for a challenging infant.