In the present investigation, single step, economic and environmentally friendly preparation of ZnO-MgO nanocomposites (ZnO, Zn 25 Mg 75 O, Zn 50 Mg 50 O, Zn 75 Mg 25 O and MgO) as photocatalyst using marine brown alga (Turbinaria ornata) as a stabilizing agent is carried out. The green route synthesized nanocomposites were analysed using various spectroscopy and microscopy techniques. The efficiency of the prepared nanocomposites materials were assessed by photocatalytic degradation of Rhodamine B (RhB) as model dye under visible light illumination. The nanocomposites displayed an improved visible light induced photocatalytic degradation of RhB. Among which, Zn 50 Mg 50 O nanocomposite sample exhibited better photocatalytic degradation performance than others samples (ZnO, MgO, Zn 25 Mg 75 O and Zn 75 Mg 25 O) which might be due to the positive synergistic effect and also by a reduced recombination of charge carriers. Further, the transient photocurrent and electrochemical impedance spectroscopy (EIS) analysis revealed that the Zn 50 Mg 50 O nanocomposite has quicker migration of light induced charge-carriers. . (a) Transient-photocurrents and (b) Nyquist impedance plot of pure ZnO, MgO and Zn 50 Mg 50 O nanocomposite samples 2 3 4 5 6 7 8