Existing research has found that individuals with higher relationship quality in intimate relationships tend to exhibit higher levels of tacit understanding and better interpersonal coordination in social interactions. However, the cognitive neural mechanisms between intimate relationship quality and interpersonal coordination are unclear. We thus argued that interpersonal neural synchronization during interpersonal interactions is an important mediator of intimacy quality and interpersonal coordination. To test this hypothesis, interpersonal neural synchrony and interpersonal coordination were measured in heterosexual dating couples during the completion of a jigsaw puzzle interaction task using an EEG-based hyperscanning technique. A total of 33 couples’ data were included in the analysis. The results showed that β-band synchronization in the left sensorimotor cortex of both members of a romantic couple fully mediated the relationship between intimacy quality and interpersonal coordination during the jigsaw puzzle task. In addition, we found that intimacy quality significantly predicted interpersonal neural synchrony in a jigsaw puzzle task with interaction but not in a jigsaw puzzle task without interaction. These findings support the idea that intimacy is a result of interindividual interactions and that interpersonal neural synchronization during interactions plays an important role in relationship quality and interpersonal coordination.