Concrete is a fundamental construction material, but its production contributes significantly to carbon emissions. To mitigate this impact, alternative materials like seaweed ash have been explored as partial cement replacements. There are, however, gaps in the literature to understand the exact influence of the ash of seaweed at its various cement replacement levels. This study tends to find out the effect of seaweed ash in concrete strength at a selected level of replacement (0%, 5%, 10%, 15%). The impact was evaluated against compressive strength (CS), split tensile strength (STS), and flexural strength (FS). It was found that 15% replacement of OPC with seaweeds exhibits the maximum mechanical strength in contrast to other replacement levels. Response surface methodology (RSM) was used to model each strength parameter, giving the predictive equation of the impact of seaweed ash in the concrete. These results reveal a gradual improvement in CS, STS, FS with respect to the addition of seaweed ash. The same leads toward revealing deeper insights related to the prospective contribution of seaweed ash as a sustainable material toward concrete practice.