Corporate governance is central to effective corporate control, accountability and protection of stakeholders' interests. This study examines the relationship between selected corporate governance characteristics and financial performance of quoted non-financial institutions in Nigeria, with particular attention to board size, board independence, gender diversity and the moderating role of firm size. The study adopts an ex-post facto design and uses secondary data obtained from the annual reports and financial statements of 37 quoted non-financial institutions over 2017–2023, producing 259 firm-year observations. Financial performance is measured using return on equity (ROE), while board size, board independence and gender diversity represent key governance variables. Firm size is measured using the natural logarithm of total assets. Directors' independence and audit committee size are included as additional governance variables in the regression model. Data are analysed using descriptive statistics, correlation analysis, variance inflation factor and ordinary least-squares regression. The results show that board size has a significant negative relationship with ROE, while board independence has a positive and statistically significant relationship with ROE. Gender diversity has no statistically significant direct relationship with ROE, Furthermore, interaction of gender diversity with firm size is statistically significant, indicating a moderating effect. The findings suggest that effective board structure and independence are important for improving financial performance, while the value of gender diversity may depend on firm size and organisational context.