Carrageenan is a high-in-market demand product extracted from Kappaphycus alvarezii, but it generates substantial solid waste as a by-product, at around 65-70%. Carrageenan solid waste (CSW) contains high cellulose, which is lighter in weight but stronger in bond characteristics. This makes CSW suitable as a substitute material for sand to develop cellular lightweight concrete (CLC). Meanwhile, fly ash (FA), a charcoal by-product, can be utilized as a substitute cement material due to its high silica and alumina content. Combining CSW and FA can result in a sustainable alternative material. The objective of this study is to develop and characterize CLC using a combination of CSW and FA. The finding shows that the combination significantly affected (p<0.05) the physico-mechanical properties of the compressive strength, split tensile, and water absorption, but there is no significant difference (p> 0.05) in the specific gravity. The findings also shows that the best combination of CSW and FA is the one with a ratio of 100% and 20% (T8), which results in the following characteristics: 0.783 kg/cm3 of the specific gravity, 1.231  0.07 MPa of the compressive strength score, 0.171 ± 0.03 MPa of the split tensile score, and 47.67 ± 0.58 % of water-absorption percentage value. Furthermore, the study suggests that more ratio of Cement should be increased to higher the physico-mechanical properties of CLC. Future research should be conducted such as Durability, thermal conductivity, or weathering resistance and practical application as non-structural CLC.