In this study, the effects of liquid extract from brown seaweed Colpomenia sinuosa on growth rate and biochemical composition of Treptacantha barbata were investigated. Liquid extract of C. sinuosa was used in two different concentrations (1ml l -1 (C1), 2 ml l -1 (C2)). Also, the Conway medium was used in the control group. The experiments were carried out for 35 days, and the highest growth rate and biomass yield was determined in C1 (p<0.05). The crude lipid of the groups varied from 1.18±0.03% ile 2.19±0.06%. The highest crude protein content was found in the control and C1 groups. It was determined that there are statistically significant differences between the ash contents of all groups (p<0.05). Our results showed that saturated fatty acids (SFA) constituted more than half of all groups' fatty acid contents. Monounsaturated fatty acids (MUFA) content of groups varied between 23.51 ± 0.23% and 25.36 ± 0.22%. In the study, it was determined that the C1 group had higher polyunsaturated fatty acid content compared to other experimental groups. As a result of the study, it was determined that C. sinuosa liquid extracts could be used as a nutrient medium in T. barbata cultivation.