Modular steel structures (MSS) are distinguished from traditional steel structures (TSS) by the grouping and discontinuous features of columns, inter-modular connections (IMC), and other structural components. Vertical assembly requires shear-keyed grouped IMC to support modules' tubular columns, resulting in columns and IMC clusters that complicate force transfer. This study reported experimental, numerical, and analytical investigations on the compressive behaviors of steel shear-keyed grouped tubular columns. Four large-scale tubes with varied shear-key heights (Lt) and thicknesses (tt) were subjected to axial compression testing. The test results demonstrated that raising Lt and tt increased the buckling resistance of the tubes but lowered the ductility. The failure was caused by S-shaped local inward and outward buckling by neighboring columns located at shear keys, mid-height, or between 1/4 and 1/2 the tube's height. The finite element model (FEM) was generated to study the effects of 9 parameters using 147 models. The impact of tube spacing and numbers, varying shear-key length (d), width (b), Lt and tt, tubes length (D), width (B), thickness (tc), and height (Lc) on compression behaviors were observed. The results show that the nominal strength of neighboring tubes was reduced to achieve compression yielding and underwent local elastic buckling, making the EC3:1-1 Class 3 slenderness limit non-conservative. Prediction equations in EC3:1-1, CSA S16, AISC360-16, and GB50017 were used to evaluate the ultimate compressive resistance (Pu) of shear-keyed grouped tubes, but they overestimated results, proving non-conservative. To assess compressive behavior conservatively, modified prediction equations were proposed. Reliability analysis on 133 models showed that they accurately predicted the axial compression behavior of steel shear-keyed grouped tubular columns and can be used for MSS design.