In massive multiple-input multiple-output (MIMO) systems, deploying a high-resolution analog-to-digital converter (ADC) for each antenna element may not be power and cost efficient. Assuming each antenna is connected to a pair of one-bit ADCs, i.e., one for each real and imaginary component of the received signal, a near maximum likelihood (nML) detector for uplink multiuser massive MIMO systems is proposed in this paper. The proposed nML detector is based on a standard convex optimization technique and becomes optimal when the number of receive antennas at the base station goes to infinity. Simulation results show that the proposed nML detector can simultaneously support multiple users using higher-order constellations, e.g., 16 quadrature amplitude modulation, and performs much better than a simple zeroforcing (ZF)-type detector.