In this study, the details of a new connection based on a steel box and an optimized arrangement of internal and external stiffeners were presented. This configuration enhances the flexural and shear performance of the connection by providing appropriate force-transfer paths and improving stress distribution, while eliminating the need for continuity and doubler plates and simplifying the fabrication process. In the first step, three superior specimens were selected based on preliminary numerical analyses and evaluated under laboratory cyclic loading. The outputs included elastic stiffness, ultimate moment, energy dissipation capacity, panel-zone rotation, and strain in the main components. Based on the experimental results, the best specimen was identified, and after validating the numerical model accordingly, a parametric study was conducted to examine the effect of the steel-box thickness on the connection behavior. The objective was to analyze the sensitivity of the results to variations in the box thickness and assess its role in improving connection rigidity. Experimental results showed that the model equipped with internal stiffeners aligned with the beam flange increased stiffness and strength by factors of 9 and 5.2, respectively, compared with the baseline model. Moreover, adding external stiffeners improved the ultimate moment and stiffness by factors of 3.8 and 6.2, respectively. Numerical results indicated that when the steel-box thickness is selected at least 2 mm greater than the beam-flange thickness, the strength and stiffness reach a stable condition, and further increases in thickness have negligible effects. Numerical findings also showed that for thicknesses greater than or equal to 8 mm, the connection achieves ductility of about 7.4, and its equivalent damping ratio increases by up to 40%.