Magnesium ion conduction in poly(ethylene oxide) (PEO) based polymer electrolyte incorporated with room temperature ionic liquid (RTIL) is reported. The electrolyte films comprise the PEO complexed with magnesium trifluoromethanesulfonate (or magnesium triflate) added with different amount of ionic liquid, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMITf). The polymer electrolyte with ∼50wt.% of ionic liquid offers a maximum electrical conductivity of ∼5.6×10−4 Scm−1 at room temperature (∼25°C) with improved thermal and electrochemical stabilities. The Mg2+ ion conduction in the PEO-complex is confirmed from cyclic voltammetry, impedance and transport number measurements. A significant increase in the Mg2+ ion transport number ( t M g 2 + ) is observed with increasing content of the ionic liquid in PEO–Mg salt complex and the maximum value is obtained to be ∼0.45 for ∼50wt.% of ionic liquid. The interaction of imidazolium cations with ether oxygen of PEO, as evidenced from FTIR and Raman studies, play an important role in the substantial enhancement in the t M g 2 + value.