Subnational constitutions are defined as basic documents for given subnational entities which lay down entrenched basic rules on subnational identity, representative structures, organization of powers, fundamental rights and/or policy principles, and require the approval of the people or representatives of the subnational entity. The strength of the power to adopt subnational constitutions, both as to content and procedure, is measured and situated against the background of federal design and societal context. It is hypothesized that the power to adopt subnational constitutions is strong in traditional federations with pre-existing constitutions, and weak (incomplete, restricted and under central oversight) and contested in fragmenting multinational federations. It is also held that in the latter case, self-constituent capacity can be secured through the entities’ involvement in national constitutional arrangements. The Belgian case is used to illustrate the hypotheses.