β-Ga2O3 is an ultrawide-bandgap semiconductor with exceptional breakdown strength, high critical electric field, and scalability, making it a promising material for next-generation power and RF electronics. However, its intrinsically low thermal conductivity and strong self-heating effects remain major challenges to reliable operation. This work develops a physics-based compact model for β-Ga2O3 MOSFETs that is suitable for circuit-level simulation and design exploration. The model is calibrated using experimentally validated TCAD simulation results and captures key physical phenomena, including quantum confinement, electron centroid shift under bias, self-heating, and parasitic resistances. With a small set of physically meaningful parameters, the proposed model achieves geometry scalability and enables predictive simulation across a wide design space. Comprehensive validation against both TCAD and experimental data demonstrates excellent agreement for I–V characteristics, transconductance, and output conductance. The developed model provides a compact yet accurate framework for understanding electrothermal interactions in β-Ga2O3 devices and supports circuit designers in optimizing performance, efficiency, and reliability.