This paper describes an experimental and numerical study of the hysteretic behavior of side-reinforced joints of a steel frame manufactured from Q345B steel and welded using an E5015 electrode. The objectives of this study were to observe the mechanical behavior of side-plate reinforced joints under cyclic loads; identify their plastic hinge, skeleton curve, stiffness degradation, ductility, and energy dissipation capacity; and provide useful test data for future damage analysis of steel frames with side-plate reinforced beam–column connections. Two specimens were designed, and both were tested under cyclic loads. The test setup consisted of one beam at the end and a column connected by three groove welds. The cyclic loads were applied to the beam’s free end, and the sizes of the beams and the columns of the two specimens were held constant because the only element studied in the present work was the side-plate reinforcement. In this paper, the responses of these two joints are discussed in terms of their experimentally and numerically obtained failure modes, hysteretic curves, plastic hinge, skeleton curve, stiffness degradation, ductility, and energy dissipation capacity. The results show that the steel frame’s side-plate reinforced joints, compared with normal joints, have smaller plastic hinge displacement and deformation, and higher bearing capacity. Furthermore, if designed according to code, all these welded joints perform satisfactorily.