Numerous studies have found that nature sounds have a positive effect on individuals, but the effect of nature sounds on individuals’ social interaction behaviours has not been investigated. This study conducted a soundscape intervention experiment in an urban park activity space and covertly observed people’s social interaction behaviours in response to varying sound conditions. In the experiment, birdsong and water sounds were introduced as intervention sounds to alter the acoustic environment. Three variables were defined to represent the effects of the interventional sounds on social interactions: the proportion of people engaged in social interactions, the frequency of social interactions, and the proportion of total time spent on social interactions. The results revealed that the intervention of birdsong and water sounds increased the percentage of people involved in social interactions compared to the control group. The sound of the water also promoted time spent on social interactions. In addition, the positive effects of natural sounds primarily promoted group interactions compared to one-to-one social interactions. This study provided evidence that nature sounds can encourage social interaction, and the results may contribute to the refinement of social interaction-friendly space design theory and the promotion of long-term social cohesion.