Observational fear learning is common in children as they learn to fear by observing their parents. Although adaptive, it can also contribute to the development of fear-related psychopathologies such as anxiety disorders. Therefore, it is important to identify and study the factors that modulate children’s sensitivity to observational fear learning. For instance, observational fear learning can be facilitated by the synchronization of biological systems between two people. In parent–child dyads, physiological concordance is important and varies according to the attachment relationship, among others. We investigated the joint effect of parent–child physiological concordance and attachment on observational fear learning in children. A total of 84 parent–child dyads participated in this study. Parents were filmed while exposed to a fear-conditioning protocol, where one stimulus was associated with a shock (CS+) and the other was not (CS−). This recording was then shown to the children (observational learning). Thereafter, both stimuli (CS+ and CS−) were presented to the children without any shock (direct expression test). For both the parent and child, skin conductance activity was recorded throughout the entire procedure. We measured physiological concordance between the parent’s phasic skin conductance signal during conditioning and the child’s signal during the observational learning stage. Children showing stronger concordance and a less secure relationship with their parent exhibited higher levels of fear to the CS+, as indicated by a heightened skin conductance response during the direct expression test. Thus, when children have an insecure relationship with their parent, strong physiological concordance may increase their sensitivity to observational fear learning.