Round-ended concrete-filled double skin steel tubular (RCFDST) columns are innovative composite columns consisting of two steel tubes filled by concrete in between them. The external round-ended tube is formed from two flat plates with two round ends welded longitudinally at the other two sides. This enables simple fabrication and installation of beam-to- column connections in the flat portions of the cross-section compared with the case of circular columns, while the round-ends add a beautiful curvilinear appearance. The internal tube is made of a rectangular section added mainly to reduce the weight of the column. There have been no applications of RCFDST columns with inner rectangular tubes worldwide due to the lack of design provisions. Accordingly, this paper addresses experimentally the axial compressive performance of these columns. Three width-to-depth (B/D) ratios of the outer tubes are considered varying between 1.6 and 2.5. From this investigation, the effect of increasing the B/D ratio has been found to reduce the passive confinement effect of the outer steel tubes on the concrete core. However, it is observed that the failure modes for the current RCFDST columns are typical to those of different cross-sectional shapes of the concrete-filled single- and double-skin steel tubular (CFST and CFDST, respectively) columns, from which the local buckling of the external steel tubes takes place associated with concrete crushing. Additionally, the shear failure of the concrete core is also observed in specimens with large B/D ratios. The columns' strengths have been compared with the modified predictions (through considering the contribution of the inner tubes) of the European standards and available proposals in literature, which are originally recommended for CFST columns. A suggested strength is given at the end.