Due to their excellent biaxial bending strength, box columns are widely employed in countries with high seismic risk such as Iran and Japan. Although extensive research into the moment connections between I-beams and wide-flange columns has been conducted, a review of the literature indicated that studies on I-beam to box column moment connections are insufficient. To fill this gap, the cyclic behaviour of a two-sided moment connection of an I-beam to built-up box column was studied in this work. Non-linear finite-element models of the connection were developed and validated using test results available in the literature. In the studied connection, the beam flanges are joined to the column flange through flange plates. To reinforce the connection, vertical top and bottom rib plates were welded to the column flange and the flange plates. A parametric study was carried out to investigate the influences of the geometry of the rib plates on the behaviour of the connection. The results indicated that rib plates decrease the stress on the groove weld between the flange plates and the column flange and have insignificant influences on the global responses of the connection.