The research addresses the effect of diagonal and horizontal stiffeners on shear transfer in rigid frame square knees. Rigid frame square knees are an integral component of pre-manufactured metal building systems. This paper examined a more efficient design of the rigid frame square knee. Five full-scale laboratory tests on square knee joints were performed to verify the effects of a diagonal stiffener on shear transfer. Two frames were fabricated without diagonal stiffeners, and three frames were fabricated with 1/8- inch- thick diagonal stiffeners, not extending the full diagonal length of the knee web. Experimental results, coupled with a finite element analysis, are compared to AISC provisions in Section F4: shear yielding and shear buckling, and in Section G3: tension field action. This research shows that if diagonal stiffeners are needed, then thin, shortened diagonal stiffeners are sufficient to restrain shear buckling of the knee web until shear yielding occurs.