The application of concrete-filled double-steel tube (CFDST) joints in structural engineering has received great attention due to their high load-bearing capacity, excellent ductility, and favorable seismic performance. In this study, the load-bearing capacity of CFDST brace-column K-joints under monotonic loading was examined using the finite element method via Abaqus. The numerical results were successfully validated against experimental and analytical models from the literature, showing good agreement in load-displacement response and failure modes. A parametric study was conducted to investigate the effects of brace diameter, column-brace angle, and sandwiched concrete between the inner and outer steel tubes on the failure behavior and overall capacity of the K-joint. Results indicate that the load-bearing capacity decreases with increasing brace diameter and with increasing brace-column angle. Models with sandwiched concrete exhibited significantly higher peak loads compared to hollow joints, due to the composite action enhancing stiffness and delaying local buckling. The maximum load resisted by the CFDST K-joint reached 2755 kN, representing a 156% increase over the joint without concrete. Although concrete infill substantially improves stiffness and capacity, the compressive strength level of the concrete plays a negligible role; thus, even low-strength concrete can be effectively used. Comparison of ultimate displacements also revealed that concrete-filled models possess much greater ductility than hollow ones. The novelty of this study lies in the simultaneous parametric investigation of brace diameter, brace-to-column angle, and concrete infill on the load-bearing capacity of CFDST K-joints using a validated finite element model.