Raising a child with an autism spectrum disorder is a challenging experience that can impact maternal well-being. Using a daily diary methodology, this study investigates (1) the relationship between stress and negative affect, and (2) the role of daily positive affect as a protective factor in the stress and negative affect relationship. Results from hierarchical linear models revealed that higher levels of stress were associated with decreased negative affect, both within and across days. Daily positive affect buffered the immediate and longer-lasting negative impact of stress on days of low to moderate levels of stress. Implications of the present study are discussed with regard to theoretical models of positive affect, the development of intervention programs, and directions for future research.