Fossil-free ironmaking is indispensable for mitigating the massive CO₂ emissions from the steel industry, accounting for ~7% of the total CO₂ emissions and thus being the largest single cause of global warming. Hydrogen-based direct reduction (HyDR) of iron oxides is one of the most promising techniques in that context. HyRD by nature is a multistep solid-gas reaction, involving several complex phenomena where microstructure plays a key role, such as non-volume conserving phase transformations, mass loss and transport, and the local accumulation of gas inside of the material and associated volume expansion. All these can lead to delamination, cracking, and pore formation. However, a 2D post-mortem microstructural analysis is strongly affected by sample preparation, which makes it unsatisfactory to reveal the actual porosity evolution, especially the connectivity of the 3D pore networks. Yet, this information is crucial to better understand their role in the outbound mass transport (of oxygen and of water). In this study, we employed in-situ synchrotron X-ray nano-tomography, enhanced by phase contrast, to characterize the porosity during HyDR of hematite. The time-resolved tomography scans show the formation and evolution of the pores and the associated network as well as percolation features. The 3D quantification of the porosity, in terms of size, morphology, spatial distribution, and connectivity were conducted using these data. The pore formation mechanism and the correlation between porosity evolution and reduction kinetics during HyDR are discussed.