Replacing our fossil fuel dependency with renewable energy requires scarce metals found on the deep-seabed floor in areas beyond national jurisdiction. These can be obtained through deep-seabed mining (henceforth referenced to as DSM). However, significant biodiversity can be found here, which produce ecosystem services that are critical to the health of the oceans. Although there is limited knowledge on the short-term and long-term effects of DSM on biodiversity, initial research has shown it to be detrimental to biodiversity and result in permanent habitat destruction. Therefore, the International Seabed Authority (ISA) is mandated to organize and regulate all forthcoming deep-seabed-related activities whilst ensuring that the marine environment is protected from harmful effects that may arise from it. However, the ISA’s legtitimacy and accuntability should systematically be reviewed to ensure good and effective governance practises over DSM in areas that are common heritage of humankind. This research seeks to invevstigate this by interviewing 10 relevant stakeholders and asking questions in line with the following sub-criteria: input legitimacy, throughput legitimacy, output legitimacy, information disclosure and addressing concerns. According to the interviewees, the ISA scored medium in most sub-criteria and low in information disclosure. Thus, although improved, it was found that relevant stakeholders do not yet perceive legitimacy and accountability as satisfactory with regards to DSM, which is severe given that the ISA is charged to act on behalf of humankind. Many reasons and accompanying examples were given for the low scores. These included inadequate quality of the website and DeepData database, bias of the Secretariat and secrecy surrounding the procedures of the Legal and Technical Commission. This means that the ISA is under capacitated and in need of reform. Especially considering the differing societal needs and scientific information available regarding biodiversity in the deep-sea since the ISA’s set-up in 1982. These findings indicate and explain the ISA’s shortcomings which provides optimal opportunities for reform of the ISA. Also, this research fuels current debates in a positive manner, which might make people more aware of this complex yet crucial sustainability issue that will partially determine future energy transitions.