Wide-bandgap (WBG) and ultrawide-bandgap (UWBG) semiconductors are key materials for high-power electronics, radio-frequency electronics, and ultraviolet optoelectronics. However, heterogeneous integration remains challenging because conventional heteroepitaxy is constrained by lattice mismatch and differences in thermal expansion coefficients, while interface states and non-ideal metal–semiconductor contacts can degrade carrier transport and device performance. Two-dimensional (2D) materials offer an important alternative approach to heterogeneous integration. Their dangling-bond-free surfaces and van der Waals interactions relax lattice-matching requirements and facilitate membrane release, while their tunable electronic properties enable control over band alignment and interfacial carrier transport. This review examines the integration of 2D materials with WBG and UWBG semiconductors, focusing on epitaxial growth, interface physics, optoelectronic devices, and scalable integration. Representative 2D material-assisted epitaxial modes include remote, pinhole, and van der Waals epitaxy. Mixed-dimensional heterostructures are discussed in terms of their interfacial electronic properties, contact engineering, passivation strategies, and thermal transport. Photodetection represents an important optoelectronic application of these heterostructures. Taken together, these advances highlight the growing importance of 2D materials in WBG and UWBG integration and point to broader opportunities in future electronics.