Hydrogen-assisted fatigue in pipeline steel has become a major concern in the field of energy transition. In recent decades, growing awareness of environmental issues has significantly increased interest in hydrogen as an alternative energy source. Hydrogen is affordable and can be transported using the existing natural gas infrastructure. However, the structural integrity of pipeline steels, especially under hydrogen-assisted fatigue conditions, poses significant challenges to the durability and safety of modern energy infrastructure. Since fatigue is a common cause of pipeline failure, a deeper understanding of hydrogen’s impact on the fatigue behaviour of these pipeline materials is essential. This project investigates the fatigue crack growth (FCG) behaviour of API X65 pipeline steel and its welding material in hydrogen enriched environments. To achieve this, a modular hydrogen autoclave testing device was designed and implemented to conduct in-situ hydrogen gaseous fatigue tests on eccentrically single-edge notched tension (ESE(T)) specimens. The fatigue crack growth relationship (da/dN vs. ∆K) was measured, and hydrogen-assisted fatigue crack growth was observed and compared to benchmarks in nitrogen and air. The study employs two testing frequencies to examine the influence of loading frequency under constant load and hydrogen pressure. This approach evaluates the combined effects of hydrogen and the microstructures of both base and weld metal. The investigation focuses on material responses in terms of crack propagation rates, measured using direct current potential drop, and fractographic analysis. To determine the required hydrogen pre-charging time, a hydrogen diffusivity FEA (Finite Element Analysis) modelling approach was used to estimate the hydrogen equilibrium concentration in the specimens before testing. The experimental setup involved pre-charging the specimens in a hydrogen environment at 10 MPa, followed by fatigue testing at frequencies of 0.1 Hz and 10 Hz. The findings indicate that the hydrogen environment accelerates fatigue crack growth compared to inert gases and air. The base metal (BM) at 10 Hz showed approximately two times greater susceptibility to hydrogen-assisted cracking compared to the weld metal (WM). At higher ∆K values (∆K > 50 MPa√m), the crack growth rates of base metal (BM) and weld metal (WM) were similar. However, at lower ∆K values (20-30 MPa√m)), the BM exhibited about one order of magnitude higher crack growth rates. The study emphasizes the importance of microstructural characteristics, including grain boundaries, size, and micro-hardness, in affecting fatigue crack growth. The study confirms that loading frequency significantly influences H-AFCG rates, where at loading frequency f = 0.1 Hz, FCGR increases by 2.3 times for BM and 3.9 times for WM compared to results at 10 Hz frequency, with lower frequencies allowing more hydrogen diffusion and promoting the transition from ductile to brittle cracking. Further findings indicated that finer grains in WM enhance hydrogen fatigue resistance due to increased grain boundary densities. However, this advantage diminishes at lower frequencies where hydrogen diffusion is enhanced, increasing susceptibility to hydrogen-assisted fatigue crack growth (HA-FCG). Analysis of fracture surfaces reveals a significant transition from ductile to brittle fracture modes when moving from inert environments (air or nitrogen) to hydrogen. In hydrogen environment, the presence of quasi-cleavage and intergranular facets was prevalent, compared to ductile transgranular features in air or nitrogen. Additionally, in hydrogen environment, secondary microcrack branching a characteristic of ductile fracture is notably absent, especially at lower stress intensity factors (∆K).