Stress exposure increases risk for depressive symptoms. However, there are substantial individual differences in affective responses to stress. High-frequency heart rate variability (HF-HRV), a marker of vagally-mediated parasympathetic activity, has been conceptualized as a psychophysiological index of emotion regulation that may moderate individuals\' responses to stress. Using a daily diary design, we tested whether individual differences in resting HF-HRV moderated the association between daily child-related stress and negative affect among a sample of 84 heterosexual couples with preschool-aged children. After controlling for participants\' age, gender, socioeconomic status, employment status, and ethnicity, hierarchical linear modeling revealed that resting HF-HRV moderated both the between-person and within-person associations between self-reported child-related stress and daily negative affect. Between-person analyses indicated that the strength of the positive association between mean daily child stress and negative affect across the daily diary period increased with decreasing resting HF-HRV. Similarly, within-person analyses indicated that on days when participants reported more child-related stress than usual, the magnitude of the increase in negative affect on that day was inversely related to resting HF-HRV. Taken together, these findings suggest that lower resting HF-HRV may index vulnerability to stress-related disturbances in negative affect. This increased negative affective response to daily stress may be one pathway through which individuals with lower resting HF-HRV are at increased risk for depressive symptoms over time.