As a primarily synchronous social media form, social live streaming services offer real-time interaction between streamers and viewers, and among viewers. Users' value cocreation has become increasingly crucial for platform businesses to increase their competitive advantage. However, the previous studies using the microfoundations approach have only confirmed the employees' efforts to adopt technology as a way to achieve the firms' goals. In this article, we explore the microfoundations of external actors' (viewers and streamers) value cocreation on sports live streaming platforms (SLSPs). Taking China Sport as a case study, this article conducts netnography research with observations made of four live-streamed matches on the final matchday of the International Table Tennis Federation World Tour Grand Final 2019. In total, 16 204 real-time messages and 5540 gifting messages are reviewed. In-depth interviews are also conducted with 5 streamers and 15 viewers. As a result, a typology of viewers (managers, fans, and audiences) emerges, and five viewer–streamer–viewer value cocreation activities are revealed. Furthermore, the unique value-in-use among streamers and viewers in different activities is found. This study presents a model to show that the viewers' engagement and the value cocreation activities between viewers and streamers at a microlevel determines the value-in-use formation, which, in turn, contributes to the competitive advantages for SLSPs at a macrolevel. This study contributes to the existing literature on the engagement behavior and value cocreation by empirically examining the role of external actors' engagement as the microfoundations of value cocreation in the context of new social technologies—SLSPs.