The objective of this research is to synthesize nano sodium alginate-ZnO nanocomposite as photocatalyst to degrade methylene blue compounds. The successful synthesis of nano sodium alginate-ZnO nanocomposite was supported by the characterization of XRD (X-Ray Diffraction), FTIR (Fourier Transform Infrared), SEM (Scanning Electron Microscope) and TEM (Tunneling Electron Microscope). The first stage of the research was the synthesis of nano sodium alginate with particle size 140 nm and crystallinity index 38.78 % by XRD and TEM characterization, the obtained particle size was 146 nm × 61 nm. Nano ZnO particle size 38 nm, by XRD and TEM characterization the obtained particle size was 56.6 nm × 33.3 nm. The next stage of nanocomposite synthesis of nano sodium alginate was the impregnation with nano ZnO particle size 130 nm, by XRD and TEM characterization the particle size was 228.0 nm × 134.2 nm. The success was also supported by FTIR and SEM characterization. The final stage was the degradation of blue methylene using nano-sodium alginate-ZnO nanocomposite. The highest degradation percentage by nano sodium alginate-ZnO nanocomposite with zinc nitrate concentration of 0.075 M was 51.68% with the first order rate constant of 9.5 = 10-3 /min.