This study investigates the impact of defined benefit pension plan underfunding on the investment activities of firms listed on the South Korean stock market from 2013 to 2022. Using a sample of firms with accessible pension data, the research analyzes the relationship between pension underfunding levels and two primary types of investment activities: capital expenditures and research and development expenditures. The findings reveal that firms with higher levels of pension underfunding tend to increase their research and development spending, while there is no significant effect on capital expenditures. This indicates that firms prefer to allocate the funds saved from pension underfunding towards research and development rather than capital expenditures. This preference can be explained by the expectation that research and development investments, which are primarily funded through internal cash flows, yield higher future returns compared to investments in tangible assets or capital expenditures. Supporting studies highlight that increased research and development spending is associated with significant improvements in abnormal returns and long-term operating performance, whereas high capital expenditures can lead to negative abnormal returns. The study further explores the distinction between financially constrained and unconstrained firms in terms of their investment behavior. The positive relationship between pension underfunding and research and development expenditure is particularly pronounced in firms without financial constraints. Financially unconstrained firms are more likely to prioritize long-term growth and innovation over short-term financial stability, leading them to invest more heavily in research and development. In contrast, financially constrained firms may face limitations in their ability to invest due to higher external financing costs and limited internal cash flow, thereby prioritizing immediate financial survival over long-term investments. To investigate potential managerial opportunism in firms with underfunded pensions, the study hypothesizes that managers may leverage the flexibility and discretion they have over pension funding to divert resources to other investment activities. The findings indicate that firms with high levels of managerial ownership exhibit a stronger positive relationship between pension underfunding and research and development expenditure. This suggests that managers with significant ownership stakes have more leeway to influence investment decisions and are more likely to direct savings from pension underfunding towards research and development. The potential for managerial opportunism is further supported by the observation that such managers prioritize investments with higher expected returns, like research and development, which can enhance their own compensation and the firm's long-term prospects. The robustness of the results is tested using different measures of pension underfunding, including the ratio of pension deficits to market value and the ratio of pension deficits to cash and cash equivalents. These additional analyses consistently show that pension underfunding significantly impacts research and development expenditure but not capital expenditures, reinforcing the primary findings of the study. The implications of this study are multifaceted. First, it underscores the strategic decisions firms make regarding the allocation of resources saved from pension underfunding. The preference for research and development over capital expenditures is driven by the higher expected returns and the strategic importance of innovation and long-term growth. This finding highlights the role of internal financing in supporting research and development activities, particularly in firms with significant managerial ownership and those without financial constraints. Second, the study reveals the need for more stringent regulations to ensure efficient utilization of pension funds. The potential for managerial opportunism, where managers may use the flexibility in pension funding to their advantage, poses risks to the firm's financial health and the security of employee pensions. Policymakers should consider implementing measures that limit the discretion managers have over pension funding and enhance transparency in how these funds are allocated. Third, the results suggest that firms with underfunded pensions might be engaging in riskier investment strategies, such as increased research and development spending, to achieve higher returns. While this can lead to significant long-term benefits, it also introduces greater volatility and potential risks. Therefore, investors should be aware of the implications of a firm's pension funding status on its investment strategy and overall risk profile. In summary, this study provides valuable insights into the relationship between defined benefit pension plan underfunding and corporate investment behavior. It highlights the strategic allocation of saved resources towards research and development, the influence of financial constraints, and the potential for managerial opportunism. The findings call for improved regulatory oversight to protect the interests of employees and ensure the sustainable growth of firms. The study contributes to the broader understanding of how pension funding decisions impact corporate strategy and financial performance, offering a foundation for future research in this area.