The study investigated how legitimacy gains from environmental cost affect share prices of Nigerian Information and Communication Technology (ICT) firms. Based on the postulations of legitimacy theory, causal-comparative research strategy was applied on a population of eight (8) firms listed under the ICT sector of the Nigerian Exchange Group. Purposive sampling technique was employed to select five (5) firms that have been listed since 2013. Secondary data were extracted from the annual reports and statements of account of the selected firms from 2013 to 2022. Descriptive statistical analysis was carried out while the Least Square regression was estimated for the test of hypothesis. The finding showed that: legitimacy gains from environmental cost positively influence the share prices of Nigerian ICT firms (p-value = 0.0086). In conclusion, as environmental sustainability continues to gain prominence globally, Nigerian ICT firms that proactively invest in this area are well-positioned to reap long-term rewards in terms of both legitimacy and market valuation. Therefore, we recommend that Nigerian ICT firms should map out more investments in renewable energy sources, energy-efficient infrastructure, and sustainable product design which will help them to reduce environmental impact as well as lead to cost savings and improved financial performance in the long term.