Hydrogen-induced delayed fracture is an unpredictable and extremely hazardous failure mode that often occurs suddenly under applied stress levels significantly below a material’s yield strength. This issue must be addressed to ensure the promotion and application of advanced high-strength steels (AHSSs). The development of third-generation AHSSs has transformed automotive lightweighting strategies by achieving an exceptional balance between strength and ductility through precise microstructural engineering. A key microstructural feature enabling this balance is the coexistence of bcc and fcc phases within the steel’s microstructure. The interface between bcc and fcc phases, an important microstructural defect in AHSSs, acts as a trapping site for hydrogen atoms. However, hydrogen-trapping behaviour at the bcc-Fe/fcc-Fe interface, along with the influence of alloying element segregation, remains poorly understood. In this study, the hydrogen-trapping behaviour at Kurdjumov–Sachs oriented bcc-Fe/fcc-Fe interfaces was systematically investigated using first-principles density functional theory calculations. Parameters such as the binding energy, interfacial separation work and hydrogen Bader volume were analysed to quantify the hydrogen-trapping behaviour. In addition, the effects of alloying elements on the bcc-Fe/fcc-Fe interfacial stability and hydrogen-trapping behaviours were evaluated. The results show that hydrogen atoms preferentially segregate to the fcc-Fe side of the bcc-Fe/fcc-Fe interface with a binding energy of 34.5 kJ/mol, indicating a strong tendency for interfacial trapping. Notably, the hydrogen Bader volume was found to be a reliable indicator of hydrogen-trapping behaviour at the bcc-Fe/fcc-Fe interface. Further analysis of alloying elements revealed varying effects on the bcc-Fe/fcc-Fe interface. Nb and Mn had a minimal effect on interfacial stability. V slightly increased the interfacial separation work, improving interfacial stability. Tc, Mo, and Cr significantly increased interfacial stability by increasing the interfacial separation work. Conversely, Ni decreased the interfacial separation work, thereby weakening the interfacial stability. This study provides an atomic-scale understanding of the hydrogen-trapping behaviour at the bcc/fcc interface and offers theoretical guidance for designing the chemical composition of AHSSs with enhanced resistance to hydrogen embrittlement.