Maternal gatekeeping describes mothers' attempts to influence father involvement in child-rearing. While the effect of maternal gatekeeping on the father-child relationship has been explored, less is known about how fluctuations in the perception of maternal gatekeeping influences fathers' family outcomes and whether father's attachment might moderate these associations. Building on family systems theory, this study explores the within-person effect of negative maternal gatekeeping on father's weekly reports of romantic relationship quality and feelings of exclusion from the family and tests father attachment as a moderator of these associations. Two hundred seventy-seven Canadian and German fathers with children under the age of 6 took part in an 8-week online diary study. Data were analyzed using multilevel structural equation modeling. During weeks when fathers perceived more maternal gatekeeping than was typical, they reported lower positive romantic relationship quality, higher negative romantic relationship quality, and felt more excluded from the family system. Avoidant attachment moderated the within-person effect of perceived maternal gatekeeping on negative romantic relationship quality, such that within-person increases in perceived maternal gatekeeping were more strongly linked with higher than-average negative romantic relationship quality among more avoidant fathers. Our findings shed new light on the within-person effects of perceived maternal gatekeeping on fathers' feelings regarding the family system. (PsycInfo Database Record (c) 2023 APA, all rights reserved).