Box girders are convenient structural members for flexurally resistant frames, but due to the closed shape of box columns and the difficulty of accessing internal columns to create a reliable load path, providing proper beam-to-box column connections is still under study. To solve this problem, a new easy-to-manufacture moment connection is proposed for connecting I-beams to box columns. The connection consists of vertical plates that pass through the columns and are welded to the column flanges, and beams that connect to plates called through plates. In this paper, the force transmission mechanism of the connection is described, and a design method for sizing through plates and other related parts is presented. An experimental procedure consisting of two relatively identical cyclically loaded specimens was carried out to evaluate the seismic performance of the connection. The results show that the specimen achieves a total floor drift of at least 0.06 rad before experiencing strength degradation. In addition, the connection behavior for a range of parameters, including member dimensions and through-plate dimensions, was investigated through a validated finite element model to check the proposed basic design criteria for connections.