Experimental tests and numerical analyses were carried out to study the seismic behavior of semi-rigid partial-strength steel–concrete composite beam-to-column joints. First, a detailed description of the experimental program and the main results of the experimental tests were presented. Four full-size sub-assemblages representing exterior composite beam-to-column joints were tested under cyclic loading at the Laboratory of the Politecnico di Milano. The specimens exhibited large dissipative capacities and very stable and ductile behavior without significant reduction in strength and stiffness. The hysteretic behavior and the dominant failure modes of the tested sub-assemblages, the contribution of the column web panel to the overall joint rotations and the mechanisms which may describe the force transfer between concrete slab and column were analyzed and discussed. Then, finite element models of the tested specimens were developed and numerical analyses were carried out in order to further investigate the stress state in the composite sub-assemblages and to study the influence of various parameters on the joint behavior. The influence of the slab contribution, the mechanisms of force transfer in the slab in the connection zone, the effect of the degree of partial interaction and the stress field existing in the slab for various layout of seismic rebars were studied.