Attachment insecurity is influenced by both environmental and genetic factors, but few studies have examined the effects of gene-environment interactions. In the context of environmental stress, a functional variant in the glucocorticoid receptor co-chaperone FKBP5 gene has been repeatedly shown to increase risk for psychiatric illness, including depression. We expand on prior work by exploring cross-sectional attachment by gene effects on both attachment insecurity and downstream physiological and behavioral measures in a diverse community sample of school-aged children (N = 99, 49% girls, Mage = 10.29 years, 66.6% non-White) and their mothers. Specifically, we examined moderating effects of FKBP5 rs3800373 genotype on the links between parenting insensitivity (overcontrol) and child attachment. Further, we assessed whether FKBP5 moderates the links between maternal and child attachment and children’s emotion regulation self-report, respiratory sinus arrhythmia (RSA) in response to a standardized laboratory stressor, and depressive symptoms. Higher levels of overcontrol predicted lower child attachment security only in FKBP5 minor allele carriers. Among children with two minor alleles (CC), attachment security was negatively associated with emotion suppression, rumination, depressive symptoms, and RSA reactivity; similarly, for these children, maternal attachment anxiety was positively associated with depressive symptoms. The findings can be conceptualized in a differential susceptibility framework, where the FKBP5 minor allele confers either risk or resilience, depending on the parenting environment.