Mothers of children with an autism spectrum disorder (n = 49) participated in a 30-day diary study which examined associations between mothers' positive and negative perceptions of their children, marital adjustment, and maternal well-being. Hierarchical linear modeling results revealed that marital adjustment mediated associations between positive perceptions and maternal well-being. Mothers who reported higher levels of positive perceptions of the child were higher in marital adjustment and well-being. Results also revealed that marital adjustment moderated the relation between negative perceptions and negative maternal affect. Mothers low in marital adjustment had a positive association between negative maternal perceptions of the child and negative maternal affect. These findings highlight the dynamic roles that mothers' perceptions and marital adjustment play in determining maternal psychological outcomes.