The present study investigated underlying processes of the effect of maltreatment on psychopathology (i.e., internalizing and externalizing problems) in a group of 111 maltreated and 110 nonmaltreated 7-10 year-old children (60% boys). We tested the moderating and/or mediating roles of emotion regulation and the mother-child relationship quality (pattern of relatedness) using Structural Equation Modeling. Emotion regulation, but not the pattern of relatedness, mediated the relation between maltreatment and psychopathology. This mediation was moderated by the pattern of relatedness: For the group of children with an insecure pattern of relatedness, maltreatment was related to lower levels of emotion regulation, which was predictive of higher levels of internalizing and externalizing symptomatology. In contrast, for the secure relatedness group, there was no mediation by emotion regulation since the impact of maltreatment on emotion regulation was not significant. Implications of the mediating role of emotion regulation and the buffering role of the mother-child relationship quality were discussed.