This thesis advances a new category of self-determination: transboundary self-determination. This thesis posits that transboundary self-determination may: (1) Remedy the disruptive effects of international borders on communities between two or more States separated by an international frontier, and (2) To promote a truly international peace and security in which the rights of communities between borders are prevented from falling between the cracks of the international legal framework. The original contribution this thesis makes is the enunciation of transboundary self-determination as a remedial response to the gap in international law governing the cross-border rights of communities between two or more States. As such, it is an in-between category for communities between borders who are neither internal to the State nor external to it. I suggest that this remedial right is necessary to ensure the operative effects of uti possidetis as a “photograph” of the territorial situation; a photograph that captures more than the geometric lines of frontiers and reflects the lived experience of borderlands between two or more States. Taken together the three expressions, or trichotomy of self-determination – internal, external and transboundary – ensure that no one is left behind in the expression of self-determination. It caters to the rights of transboundary peoples, broadly defined as peoples whose self is necessarily determined through cross-border cultural, spiritual/ religious, livelihood and family ties. Had it not been for borders or territorial changes these transboundary rights would have flowed freely without territorial disruption. Rooted in the jus cogens character of the right and principle of self-determination, transboundary self-determination derives its legal and normative force from the right of all peoples to ‘freely determine their political status and freely pursue their economic, social and cultural development’ enshrined in the UN Charter. Through the eight-cases presented in Chapter 3, the thesis presents examples of defacto and dejure transboundary rights from the Irish Border, Abyei Area, Kashmir, Kurdistan, Chagos Islands, Red Sea Islands, Sami Laplands and North America. These findings suggest that while the enunciation of transboundary self-determination as a unique category is novel, the rights it encapsulates are established and suggest a rule that has long been in circulation only to find a complete expression now, in this thesis.