Built-up box sections are gaining popularity as column members in cold-formed steel (CFS) construction. Applications include CFS trusses, space frames, and portal frames. Two identical CFS unequal angles are used in this article to form built-up rectangular angle box columns (BUABC). These BUABC, which can be plain or with stiffeners (longer and shorter legs), are grooved on the faces of the columns in the major and minor axis directions and connected with self-drilling screws (fasteners) through their lips. There are no reports in the literature on the advantages of stiffeners (longer and shorter legs) provided in BUABC. This paper presents 78 new results for the BUABC as plain and with stiffeners (longer and shorter legs), including 14 test results and 64 finite element (FE) results. The axial strengths of BUABC with two stiffeners in the longer leg and one stiffener in the shorter leg showed an increase of about 28% on average when compared to the axial strength of plain BUABC, according to the experimental results. The FEA-derived ultimate loads and failure modes are in good agreement with the experimental results. The verified FE model was then used in the parametric study to investigate the effect of stiffeners (longer and shorter legs) on the axial strength of BUABC. The parametric study considered two distinct geometries with two distinct lengths (300 mm and 600 mm). The axial strengths of the stub BUABC tested against the Direct Strength Method (DSM) prediction were conservative, up to 3% on average, while they were 13% conservative for the short BUABC. This shows that the DSM prediction was more conservative, with an increase in slenderness that was governed by the modified slenderness ratio. The DSM strength predictions were less conservative for the stub BUABC that failed due to local buckling, whereas the DSM strength predictions were more conservative for the short BUABC that failed due to a combination of local and flexural buckling.