In this paper, we present a highly sensitive micro-electromechanical system (MEMS) accelerometer for sub-mG sensing resolution, where the thermomechanical noise (i.e., Brownian noise, BN) being inversely proportional to a proof mass has to be below 1 µG/√Hz (gravity acceleration G = 9.8 m/s2). To increase the proof mass, we propose the use of multiple-layered metal developed by Au electroplating. We then show an approach to design the spring constant for the MEMS accelerometer. A multilayer metal structure is used for serpentine flexures to suspend the high-density proof mass, which also enables us to obtain a high degree of freedom for the spring constant design without compromising the performance of the MEMS accelerometer. A proof-of-concept device has been fabricated, and the measured characteristics are consistent with the design values. The BN of the developed device is experimentally evaluated to be 22 nG/√Hz, which is one or more orders of magnitude lower than those of conventional MEMS accelerometers with the same capacitance sensitivity. The evaluation results confirm that the proposed device has potential for sub-mG sensing.