The purpose of this study was to shed light on the relationships between playfulness in upper elementary students and their quality of peer relationships and use of peer conflict resolution strategies. Three-hundred-and-sixty-five 5th or 6th graders in Seoul, Gyeonggi, and Gwangju areas completed paper-and-pencil questionnaires assessing these three constructs. The relationships were examined through correlation analyses, regression analyses, cluster analyses, and ANOVAs. Results were as follows: First, upper elementary students who had high playfulness showed better quality of peer relationship and used more compromise/cooperation, avoidance, dominance, and concession strategies to resolve conflicts with peers than upper elementary students who had low playfulness. Second, social playfulness, among other playfulness dimensions, was found to be particularly important in shaping the students’ quality of peer relationship while emotional playfulness was found particularly important for the students’ peer conflict resolution strategies. Third, cluster analysis identified three playfulness clusters, the ‘High,’ the ‘Moderate,’ and the ‘Low’ playfulness clusters. ANOVAs yielded results that demonstrated better quality of peer relationships and more use of compromise/cooperation, avoidance, dominance, and concession conflict resolution strategies in the High playfulness cluster than in the Moderate and the Low playfulness clusters. The importance of playfulness in upper elementary students’ peer relationships was emphasized and ways to develop upper elementary students’ playfulness were discussed.