We demonstrate a tunnel field-effect transistor based on a lateral heterostructure patterned from anLaAlO3/SrTiO3electron gas. Charge is injected by tunneling from the LaAlO3/SrTiO3contacts andthe current through a narrow channel of insulating SrTiO3is controlled via an electrostatic side gate.Drain-sourceI-Vcurves are measured at low and elevated temperatures. The transistor shows strongelectric-field-dependent and temperature-dependent behavior, with a steep subthreshold slope as smallas 10 mV/dec and a transconductance as high as approximately 22?A/V. A fully consistent transportmodel for the drain-source tunneling reproduces the measured steep subthreshold slope