This paper presents an experimental and numerical investigation into the structural performance of stainless steel beam-columns. The research focuses specifically on welded I-section members under compression and major axis bending to investigate interaction models. A total of 28 beam-column laboratory tests were conducted on conventional and laser-welded austenitic and duplex stainless steel members. The load was applied with varying eccentricity to achieve a range of loading conditions. Tensile coupon tests and initial geometric imperfection measurements were also carried out and are reported herein. Based on the results obtained, advanced finite element models were developed and validated. Further data were generated through a parametric study covering a wide range of cross-section and member slenderness, and combinations of loading. The accuracy, safety and consistency of existing provisions given in the Eurocode and relevant literature are assessed for the design of stainless steel I-section beam-columns. A detailed evaluation of these methods is conducted using reliability analysis. A unified design procedure has been identified as suitable for all investigated stainless steel grades, namely austenitic, duplex, and ferritic, as well as for both I-sections and hollow sections.