In many of our daily social interactions, we need to coordinate and to synchronize movements. Various studies have demonstrated that interpersonal movement synchronization has positive effects on cooperation and affiliation. Here, we investigated whether music as compared to a metronome can further strengthen these prosocial effects. We used a within-subjects design in which participants watched videos of two figures walking side by side – without being engaged in a motor task themselves. The participants' task was to imagine that they are one of these figures and that the other figure represents an unknown person. Manipulated factors were acoustic accompaniment (music, metronome, and silence) and synchrony (both in phase with the beat, other-figure out / self-figure in phase, and other-figure in / self-figure out of phase). Participants rated the closeness of the two figures, the likability of the other-figure, and how well they felt as the self-figure. All three ratings were higher with music compared to the metronome. Additionally, with music but not with the metronome, the likability of the other-figure was significantly lower when the other-figure was walking out of phase and the self-figure in phase, as compared to the other way around (other-figure in phase and self-figure out of phase). In conclusion, music can strengthen the prosocial effects of interpersonal movement synchronization, provided that one interacts with a person who moves in time with the beat.