The modular multilevel converter (MMC) for single stage DC-DC conversion is emerging as an alternative solution to the two-stage DC-AC-DC conversion. The AC current, that only circulates inside of the MMC, is used to balance the energy between submodules (SMs). However, a high voltage conversion ratio will result in a high AC circulating current that will require high power electronic device ratings and cause high power losses. In this paper, a multiport modular multilevel DC-DC converter, which can have one high voltage port and multiple low voltage ports, is proposed. The power can flow from high voltage to low voltage ports or in the opposite direction. Especially for high voltage conversion ratio applications, the proposed converter needs lower circulating currents compared to the single-port MMC DC-DC converter. The large filtering inductor, which blocks the AC components from presenting at the low voltage DC side, is also eliminated. The proposed topology can be used to connect renewable energy systems or energy storage systems to a medium voltage DC grid.