We report on the realization of voltage control of in-plane magnetic anisotropy at room temperature in ultrathin Fe/n-GaAs(001) Schottky junctions. Clear voltage-induced changes in magnetic anisotropy were observed in a Kerr ellipticity hysteresis loop using a lock-in modulation technique. The maximum change reached 4.5% of the saturation ellipticity under the application of a sinusoidal voltage signal of 1 V peak-to-peak in an Fe layer with a thickness of 0.64 nm. These results reveal the feasibility of controlling the in-plane magnetization process by the use of a perpendicular electric field. This can be a useful technique in ultralow power magnetization switching.