The objective of this study was to analyze the mediating role of a child's attachment to a pet dog relative to the child's attachment to their mother and their level of adjustment. A cross-sectional study was conducted with a convenience sample of 136 participants who owned one or more dogs (68 children: mean age = 9.01 years; 68 parents: mean age = 41.90 years). The children were asked to respond to items about their attachment to their mother and to their pet dog. The parents were asked to respond to items about their child's psychological adjustment. A mediation model was tested using the SPSS macro PROCESS; we hypothesized a mediating effect for a child's attachment to their pet dog on the relationship between the child's attachment to their mother and their psychological adjustment. The results showed that a child's attachment to their mother both directly (c′ = -0.242, p = 0.006) and indirectly (indirect effect = 0.084, bootstrapped 95% CI = 0.003, 0.174) predicted their psychological adjustment, the indirect effect being via the child's attachment to their pet dog. Overall, our data support that secure attachment to the mother predicts positive psychological adjustment. A more secure attachment to the mother predicts less attachment to the dog, and this may indicate that a secure attachment to the mother saturates the child's emotional and support needs, resulting in less psychological need for the dog. In this situation, the dog does not need to compensate for deficiencies in the maternal attachment figure, resulting in less attachment of the child to the dog. Finally, our data support an association between attachment to a pet dog and psychological adjustment in middle childhood, indicating the potentially protective role of a relationship with a pet dog.