Although research has demonstrated a relationship between maternal depression and child attention deficit/hyperactivity disorder (ADHD); the strength of the relationship is currently unclear. The results of some studies have found a strong association between maternal depression and child ADHD, while other studies have found little or no association. A meta-analysis was conducted to clarify the strength of the association between maternal depression and child ADHD. The current study included 51 published and unpublished studies that included a quantitative comparison between maternal depression and child ADHD. Mothers of children with ADHD had significantly higher levels of depressive symptoms than mothers of children without ADHD (d = 0.58; 95% CI [0.43, 0.74]; p < .001; k = 18). The relationship between maternal depressive symptoms and child ADHD symptoms was moderate (r = .22; 95% CI [.17, .28]; p < .001; k = 21). 17.11% of mothers of children with ADHD currently met criteria for a depressive disorder (95% CI [11.95, 23.89], p < .001, k = 19). The DSM version used to evaluate child ADHD status was the only moderator that was statistically significant (p = .021, k = 15). Specifically, studies that used the DSM-III or DSM-III-R were associated with larger effect sizes than studies that used the DSM-IV or DSM-IV-TR. The results suggest that clinicians should screen for the possible co-occurrence of maternal depression when working with families of children with ADHD.