Surface and borehole gravity data contain complementary information. Thus, the joint inversion of these two data types can help retrieve the real spatial distributions of density bodies. When a sharp boundary exists between an anomalous density body and its surrounding rock, the interface recovered by smooth inversion with Tikhonov regularization is not clear, leading to difficulties in the subsequent geological interpretation. In this work, we develop a joint inversion of surface and borehole gravity data using zeroth-order minimum entropy regularization. The method takes advantage of the complementary information from surface and borehole gravity data to enhance the imaging resolution of density bodies. It also produces a focused imaging of bodies through the zeroth-order minimum entropy regularization without requiring a preselection of a proper focusing parameter. We apply the developed joint inversion approach to three different synthetic data sets. Inversion results show that the focusing inversion with the zeroth-order minimum entropy regularization provides a good description of the true spatial extent of anomalous density bodies. Meanwhile, the joint focusing inversion reconstructs a more reliable density model with a relatively high resolution when a density body is passed through by one or more boreholes.