The purpose of this study was to examine the relationship between debt and the value and excess value of diversified firms in Nigeria. This is against the background that there is no consensus on the factors that distinguish value-creating from value-destroying diversified firms. Data were collected from the annual reports of 62 diversified firms listed on the Nigerian Stock Exchange between 2008 and 2018. The multilevel generalized method of moments technique in the REndo package in the R Statistical Package was used to test the hypotheses. There is a significant positive relationship between debt and diversified firms’ value and excess value. The study and its findings are significant in several ways. First, no study has examined the relationship between debt and the value and excess value of diversified firms in the Nigerian context, and the few relevant studies have been in the context of developed countries. Second, it contributes to the literature on the valuation implications of firm diversification by providing some insight into the diversification discount often documented in the literature. These findings suggest that diversified firms’ failure to use the debt capacity that diversification creates is one reason some experience valuation discount. Third, the study employs the multilevel GMM analytical technique to control for endogeneity in the absence of valid instrumental variables. This technique has not been used in the diversification literature. The implication of the findings is that diversified firms should use the debt capacity that diversification provides to invest in positive net present value projects. The use of debt also adds a layer of corporate governance required in an environment of increased complexity and opacity associated with diversification. Increased investment in positive NPV projects and improved corporate governance through increased use of debt will enhance firm performance and value. The increased valuation of diversified firms due to debt increases the ability of diversified firms to play their developmental role in society. Investors would also benefit more from investing in diversified firms with higher debt ratios than those with lower debt ratios, all things being equal.