The need for finding sustainable and environmentally friendly materials in the modern building industry is increasing. This research aimed to determine the compressive strength of mortar cube based on a replacement ratio of 15 by seaweed (Gracilaria species). The sample was washed until the pH value is neutral, oven-dried at 105 °C for 24-h and was burnt at 600 °C for 3-h. The seaweed granules and seaweed ashes were characterized via Brunauer Emmett and Teller (BET), Field Emission-Scanning Electron Microscope (FESEM), X-ray diffraction (XRD). The sample was then incorporated into the mortar and the compressive strength was determined at 3,7,14 and 28 days respectively. After 28 days, the compressive strength test showed that a 15 burnt sample has the highest compressive strength surpassing the control mix design. Therefore, eco-friendly seaweed ash could be a substitute as a green construction material. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021.