The medium-entropy alloy is a newly intriguing material showing superb properties. A simple, one-step method was developed to electrodeposit CoNiFe medium-entropy alloy films from a sulfate and citrate bath. The microstructure and corrosion resistance were investigated in CoNiFe films with varying deposition current densities. It shows the face-centered cubic structure with a preferential orientation. HRTEM-EDS observations further show information on the nanostructure and element distribution. The films exhibit a strong corrosion resistance in 3.5 wt.% NaCl solution. The films electrodeposited with a current density of 44.4 A/dm2 shows a low self-corrosion current density of 4.72 × 10−6 A⋅cm−2 in 3.5 wt.% NaCl solution. The corrosion mechanism was proposed in combination with electrochemical impedance spectroscopy results. The outstanding properties were attributed to the near-equimolar ternary components. The results lay a solid foundation for developing of highly oriented medium-entropy alloy films with strong corrosion resistance.