Metal-organic chemical vapor deposition (MOCVD) is still a dominant technique for growing III-nitride semiconductors; however, the complex growth kinetics and defect formation mechanisms continue to limit the required material quality and device performance. In recent years, the rapid advancement of in situ X-ray characterization techniques—especially those enabled by high-brightness synchrotron radiation—has provided unprecedented opportunities for probing real-time structural evolution during nitride epitaxy. This review summarizes the latest research progress of in situ X-ray studies of III-nitride MOCVD growth, with emphasis on the development of in situ MOCVD growth platforms, emerging X-ray methodologies, and their applications in monitoring surface and interfacial dynamics. In this review paper, we present the principles and implementation of in situ X-ray reflectivity, crystal truncation rods, grazing-incidence diffraction, and microbeam/coherent scattering techniques in nitride epitaxy. Through the case studies of GaN and InGaN, we identify key dynamical processes—including early-stage nucleation, strain relaxation, step-flow behavior, alloy segregation, and interface roughening—under realistic growth conditions. Special attention is paid to transient non-equilibrium phenomena such as compositional fluctuations and interface reconstruction in high-In content alloys, which remain inaccessible to conventional in situ probes. Furthermore, we highlight emerging trends brought by next-generation synchrotron sources, including millisecond- to microsecond-resolved measurements, nanoscale spatial mapping, and in situ coherent X-ray diffraction imaging. These capabilities are expected to provide direct atomic-to-mesoscale insights into island nucleation, step dynamics, defect evolution, and strain-composition coupling in complex heterostructures. Finally, we outline future research directions, such as integrating data-driven structure inversion, multi-scale modeling, and closed-loop “growth-measurement-feedback” control to accelerate the understanding and optimization of nitride epitaxy. This review shows that in situ X-ray technique has become a powerful and indispensable bridge between microscopic structural evolution and macroscopic device performance, and will play a key role in achieving precise, controllable epitaxy of next-generation wide-bandgap semiconductor materials.