Investment inefficiencies in listed firms around the world have led to investors demanding more transparency in segment disclosure. Transparency in corporate activities is essential for investors to determine the parameters of their stock return and investment efficiency across various segments of firms. Previous studies have primarily focused on a broader investment landscape in Nigeria, without adequate attention to the impacts of segment disclosure and cost of capital on investment efficiency of listed firms. Against this backdrop, this study investigates the impacts of segment disclosure and cost of capital on investment efficiency of firms in Nigeria, using longitudinal research design. Secondary data from 2015 to 2022 were extracted from the annual reports of 85 listed firms on the Nigerian Exchange Group (NGX), which were analysed through descriptive and inferential statistical methods. Firms that reported their business or geographic segments were purposively selected for this study. Study findings showed that the cost of capital of the examined firms has a negative and significant impact on their investment efficiency (Coefficient = -0.0268, p-value = 0.03079). On the other hand, the segment disclosure of the firms has a positive impact on their investment efficiency (Coefficient = 0.0119, p-value = 0.0303). These findings imply that improved segment disclosure and a lower cost of capital tend to improve the investment efficiency of listed firms in Nigeria. This study, thus, recommends that the management of firms in Nigeria should disclose more segment information in their annual reports. This could consequently boost investors’ confidence in the reporting practices of firms, reduce the cost of capital of firms, and improve firms’ investment efficiency.