A concept of decoupling ground is introduced in this paper to enhance the isolation of massive MIMO antenna arrays. For an array, mutual coupling between array elements can be achieved by the free-space coupling and the coupling currents flowing on the ground plane shared by elements. The isolation in this paper is improved by adjusting the shape of the ground plane under each element to make the mutual coupling from the free space and the ground plane out of phase. In this way, low mutual coupling is realized. As the first example, a single-polarization linear array with eight elements is designed, simulated, and measured to verify the concept as well as the simulation accuracy. The measurements align very well with the simulations. Another two examples of dual-polarization 2×2 and 4×4 square arrays are also given and simulated to demonstrate the effectiveness of the proposed technique. In all of these examples, the isolation can efficiently be enhanced with the DG for all the co-polarization and cross-polarization coupling paths of the massive MIMO elements. Compared with the previous literature, the arrays with the DG method can achieve either much better isolation or a much lower profile while keeping the other performance comparable.