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β-phase gallium oxide ( β-Ga2O3) has drawn significant attention due to its large critical electric field strength and the availability of low-cost high-quality melt-grown substrates. Both aspects are advantages over gallium nitride (GaN) and silicon carbide (SiC) based power switching devices. However, because of the poor thermal conductivity of β-Ga2O3, device-level thermal management is critical to avoid performance degradation and component failure due to overheating. In addition, for high-frequency operation, the low thermal diffusivity of β-Ga2O3 results in a long thermal time constant, which hinders the use of previously developed thermal solutions for devices based on relatively high thermal conductivity materials (e.g., GaN transistors). This work investigates a double-side diamond-cooled β-Ga2O3 device architecture and provides guidelines to maximize the device’s thermal performance under both direct current (dc) and high-frequency switching operation. Under high-frequency operation, the use of a β-Ga2O3 composite substrate (bottom-side cooling) must be augmented by a diamond passivation overlayer (top-side cooling) because of the low thermal diffusivity of β-Ga2O3.