Knowledge is often managed in unsustainable ways and is not accessible to all individuals and social groups. Examples include: (1) The interests of certain communities affected by environmental, health, etc. issues are not a priority for those deciding what should be researched. As an emblematic example, in the field of rare diseases, a part of society is excluded from the research benefits. (2) Publishers subject research centres to a multiplication of payments to access the knowledge they themselves have generated. (3) The open science model faces the dilemma of whether or not to protect the rights of researchers and institutions. If the potential research benefits should also revert to those who generate them and if, simultaneously, the research results should be widely disseminated, knowledge cannot be managed identically to business/industrial models. Solutions to these conflicts seek to curb the privatisation of science claiming for «public science». However, in the last decades a whole line of research and activism is developing to recover the «commons» concept, distinguished from «public goods», with relevant consequences for the sustainability and accessibility of knowledge. We propose citizen science as a powerful practice for constituting «the common», in a double sense: the research itself is configured as a «commons» while, by «acting in common», more co-responsible and inclusive communities are created. The epistemic, social, ethico-political and cultural elements associated with these practices are co-constituted, resulting in a co-responsible citizenship of a more open, sustainable and accessible research. Citizen science represents a fundamental element – although also depending on the motivations of the protagonists – for the governance of research systems, with the consequent reduction of the ethical-political, epistemic, social and economic conflicts characterising our societies. Motivations based on cooperation generate networks of reciprocity, deliberation and common action, mutual recognition, creating co-responsible and committed communities. Under this ethical-political principle, citizen science can truly constitute «the common».