This study investigates the impact of financial constraints on firms’ investment decision-making in Nigeria from 2015 to 2024. Specifically, it examines the relationship between financial constraints and investment levels, analyzes the influence of access to external financing, and assesses the effect of internal cash flow on investment decisions. Using a quantitative research design with a descriptive and explanatory approach, the study relies on secondary data sourced from the Nigerian Stock Exchange (NSE), Central Bank of Nigeria (CBN), and National Bureau of Statistics (NBS). Descriptive statistics, correlation, and multiple regression analyses were employed using SPSS and EViews to ensure robust results. The findings reveal that firms facing greater financial constraints invest significantly less, underscoring the hindrance limited finance poses to investment. Additionally, internal cash flow and external financing both positively affect investment decisions, with internal funds being more influential. Furthermore, high interest rates deter investment, while increased credit availability promotes it, highlighting the crucial role of macroeconomic conditions. Based on these findings, the study recommends improved access to finance for firms, particularly SMEs, through supportive financial policies. Firms are also encouraged to enhance internal cash flow through efficient financial management. Finally, the Central Bank of Nigeria is urged to adopt interest rate policies that promote investment while ensuring macroeconomic stability. These recommendations aim to strengthen firms’ investment capacities and contribute to Nigeria’s economic development.