Electronic and optical properties of pristine and metal-doped lithium niobate crystals are investigated by using first-principles DFT calculations. The results on optical properties suggest that there is a shift in the absorption edge towards visible region (red-shift) for metal-doped structures, in comparison with the pristine lithium niobite. A series of metals are used for doping and it is found that the absorption edge is shifted significantly to the visible region for the dopants; gold (Au), silver (Ag), aluminium (Al) and copper (Cu) due to surface plasma resonance. However, for other metal dopants like iron (Fe), manganese (Mn), molybdenum (Mo) and nickel (Ni), the absorption is slightly improved in the visible region and red-shift is observed. The bandgap of all the doped structures is found to be reduced, this might be proven advantageous for photovoltaic applications, which requires high optical absorption in the visible region. The dielectric constant and refractive index of the pristine and doped structures are also calculated and it is observed that the absorption trend is in accordance with dielectric constant. The optical absorption is enhanced in the visible region due to doping of selected metals (M = Au, Ag, Al, Cu, Fe, Mn, Mo and Ni) making M-lithium niobite a promising material for optoelectronic- and photonic-based applications.