The quest for sustainable pavement materials has led to the exploration of agricultural waste products like coconut shell ash (CSA) as potential modifiers for asphalt. This study evaluates the efficacy of CSA as a bitumen modifier in flexible pavements. Specifically, the research investigates the influence of varying CSA percentages on the Marshall Properties of asphalt concrete mixtures to determine an optimal incorporation level. Conventional 60/70 penetration grade bitumen was modified with CSA at percentages of 5%, 10%, 15%, 20%, and 25% by weight. The fundamental properties of the base materials were characterized, and the Marshall properties of the resulting asphalt mixtures were assessed. Results indicated a non-linear relationship between CSA content and performance. Marshall Stability increased to a maximum of 4.1 kN at 10% CSA content, with a value of 3.8 kN at 5% CSA; both values meet the specification requirements (≥3.5 kN) of the Federal Ministry of Works and Housing. However, stability significantly declined to 3.1 kN at higher CSA percentages (15-25%). Flow values remained within acceptable limits (2-4 mm) for mixes containing 5% and 10% CSA. The study concludes that CSA can effectively enhance the performance of asphalt concrete when used as a modifier at an optimal content of 5% to 10%, offering a promising solution for sustainable waste valorization in pavement construction.