Abstract The sustainable development goals (SDGs) introduced by the United Nations in 2015 are a framework for a sustainable world. Corporations also use the SDGs as a guideline for corporate sustainability. Based on corporate sustainability theory and legitimacy theory, the paper analyzes whether and how firms communicate about the SDGs on social media to increase their legitimacy or because they are linked to the firms' core business. The study analyzed more than 24,000 SDG-related tweets from Standard and Poor 500 companies. The results show that firms post tweets about the SDGs that are related to their core businesses and impacts. Hence, the SDGs are communicated in a way that addresses strategic corporate sustainability and social responsibility. These results contribute to the research by clarifying the role of the SDGs in regard to corporate strategy and legitimacy.