Abstract: Hydrogen-induced cracking (HIC) during welding is one of the most common defects in the welding process of ultra-high strength steels (UHSSs). The level of crack susceptibility is related to the hydrogen diffusion behavior across different zones of the welded joint, which is influenced by the microstructural transformation within the welded joint. An improved microphotography technique was employed to investigate the diffusion law of hydrogen in the welded joint when using the ER110S-G welding material and the 11Cr-7Ni martensitic welding material for welding Q1100 UHSS. A hydrogen micro-printing technique over 72 hours was used to trace the final distribution of hydrogen in the welded joint. The reasons for the differences in hydrogen distribution in the welded joint between the ER110S-G and 11Cr-7Ni welding materials were explored. The findings indicate that when the welded joint of the ER110S-G welding material is used, hydrogen predominantly accumulates in the welds and heat-affected zones (HAZ). In contrast, the 11Cr-7Ni welding material mainly accumulates hydrogen in the welds. The lower martensitic transformation temperature of the 11Cr-7Ni welding material inhibits diffusion of hydrogen from the welds to the HAZ, thereby reducing the hydrogen concentration in the HAZ. This study provides novel insights into the relationship between the weld phase transformation behavior of UHSS and the hydrogen embrittlement tendency in different zones of the welded joint.