<p>There is great variation among multistakeholder partnerships for the 2030 Agenda for sustainable development with regard to their lead actor and membership composition. While most such partnerships are led by international organizations and have state membership, others are business-led and lack involvement of states. The legitimacy of the latter have thus far received scant scholarly exploration in studies of the 2030 Agenda. This article evaluates the legitimacy of a business-led multistakeholder partnership seeking to advance the Sustainable Development Goals in the domain of aquaculture, namely the Global Salmon Initiative. We define three legitimacy standards against which this business-led partnership is assessed: procedural legitimacy, market legitimacy, and policy output legitimacy. Our analysis suggests that while the GSI benefits from procedural legitimacy through increased transparency, it is weaker with regard to accountability to external stakeholders. Rooted in the promotion of collaboration under competitive market dynamics, its market legitimacy rests on a fragile basis. While the GSI operates under the assumption of alignment between profit and sustainability, its relatively limited market coverage indicates lower market legitimacy than if a larger share of major companies in the farmed salmon industry were GSI members. The increase in ASC certification over the past ten years brings output legitimacy to the GSI in the narrow sense of policy output. However, embracing a broader mission has mixed consequences for long-term GSI output legitimacy.</p>