Photo/electrocatalytic water splitting has been considered as one of the most promising approaches for the clean hydrogen production. Among various photo/electrocatalysts, 2D nanomaterials exhibit great potential because of their conspicuous properties. Meanwhile, synchrotron-based soft X-ray absorption spectroscopy (XAS) as a powerful and element-specific technique has been widely used to explore the electronic structure of 2D photo/electrocatalysts to comprehensively understand their working mechanism for the development of high-performance catalysts. In this work, the recent developments of soft XAS techniques applied in 2D photo/ electrocatalysts have been reviewed, mainly focusing on identifying the surface active sites, elucidating the location of heteroatoms, and unraveling the interfacial interaction in the composite. The challenges and outlook in this research field have also been emphasized. The present review provides an in-depth understanding on how soft XAS techniques unravel the correlations between structure and performance in 2D photo/electrocatalysts, which could guide the rational design of highly efficient catalysts for photo/electrocatalytic water splitting.