The study examined board size and retained earnings of deposit money banks in Nigeria. The objective of the study was to ascertain the relationship between board size and retained earnings of deposit money banks in Nigeria. The study adopted an ex-post-facto research design, covering the period between 2010 and 2019. Secondary data were extracted from the annual reports and accounts of sampled deposit money banks in Nigeria. Total assets, total deposits, statutory reserves, and number of branches, were the control variables of the study. Multiple regression and covariance analysis were used for data analysis. The covariance analysis revealed that total asset (p-value < 0.05), total deposit (p-value < 0.05), and number of branches (p-value < 0.05) have a strong and positive relationship with retained earnings (80% approx., 78% approx., 64% approx. respectively). Statutory reserve (p-value < 0.05) and board size (p-value < 0.05) have a strong and negative relationship with retained earnings of deposit money banks in Nigeria with the following coefficients Statutory reserve 73% and board size 53% approx. The findings imply that as a total asset, total deposits, and the number of branches are increasing, the banks’ retained earnings also increase significantly and vice versa. On the other hand, as statutory reserve and board size are increasing, banks’ retained earnings decrease significantly. Hence, these variables can be used to predict and make decisions on retained earnings of deposit money banks in Nigeria. The study, therefore, recommends that deposit money banks in Nigeria should keep a small or moderate board size since an increase in board size affects their retained earnings negatively.