Solidarity has long been a fundamental value laying at the core of EU integration laws and policies as well as a guiding principle operating both horizontally and vertically in various regimes of international law. In the regional context of supranational cooperation, the Treaty of Lisbon has deepened international solidarity’s commitment emphasizing its role in binding together key stakeholders. Referring to Art. 80 TFEU, this paper aims at investigating the notion and normative relevance of infra-EU and extra-EU interstate solidarity with regard to the elusive regulatory framework of fair sharing of responsibilities in case of humanitarian crisis or mass migration emergencies. Theoretical prospects and paradoxes of European solidarity will be empirically addressed analysing the distributive justice implications of ECJ ‘migration jurisprudence’ on sharing practices (e.g. resettlements) grounded on mutual trust, solidarity and fairness.