The valley degree of freedom of electrons in solids has been proposed as a new type of information carrier, beyond the electron charge and spin(1-6). The potential of two-dimensional semiconductor transition metal dichalcogenides in valley-based electronic and optoelectronic applications has recently been illustrated through experimental demonstrations of the optical orientation of the valley polarization(7-10) and of the valley Hall effect(11) in monolayer MoS2. However, the valley Hall conductivity in monolayer MoS2, a non-centrosymmetric crystal, cannot be easily tuned, which presents a challenge for the development of valley-based applications. Here, we show that the valley Hall effect in bilayer MoS2 transistors can be controlled with a gate voltage. The gate applies an electric field perpendicular to the plane of the material, breaking the inversion symmetry present in bilayer MoS2. The valley polarization induced by the longitudinal electrical current was imaged with Kerr rotation microscopy. The polarization was found to be present only near the edges of the device channel with opposite sign for the two edges, and was out-of-plane and strongly dependent on the gate voltage. Our observations are consistent with symmetry-dependent Berry curvature and valley Hall conductivity in bilayer MoS212.