How do “novel” spaces of transnational law emerge rather than being captured within existing legal regimes? This article argues that processes driving how the subject matter of a transnational legal space is defined and framed and by whom are notable in mediating such outcomes. The article presents the empirical case study of global neurotechnology governance and examines socio-legal processes whereby individual and organizational actors have constructed and defended neurotechnology as a distinct space of transnational law. Here, I argue that this can be understood as boundary work, which examines discursive and spatial processes of demarcating social entities from one another. The article shows how attention to external and internal boundaries around and within a transnational legal domain can sensitize socio-legal analysis to the more emergent features of these spaces, including more subtle modes of exclusion, cooperation, and coordination. The article concludes by reflecting on how attention to processes of boundary work can enrich inquiry into, and critique of, the earliest stages of transnational legal ordering.