Recently, β-Ga2O3 solar-blind photodetectors (PDs) have been extensively investigated for a wide range of civil and military applications. Among them, the metal-semiconductor-metal (MSM) structure is one of the most popular candidates due to the merits of fabrication simplicity, the need for only one single-dopant active layer, easy integration with readout circuitry, high quantum efficiency, etc. However, there is generally a tradeoff between sensitivity and response speed due to the specific internal gain mechanism. In this work, MSM PDs based on the molecular beam epitaxy-grown β-Ga2O3 film were fabricated, and the metal/semiconductor (M/S) interfacial properties were tailored through the low-energy Ar-plasma pretreatment, resulting in the localized oxygen deficiency and a sharper interface. Accordingly, the PD sensitivity was dramatically improved, and the advantage of internal gain, i.e., high quantum efficiency, was preserved or became even larger. For example, the 60-W pretreated sample exhibited a high responsivity (R) of 8.41 A/W and a large specific detectivity (D*) of 1.24 × 1014 Jones, both increasing by one order of magnitude in comparison with the untreated sample. More interestingly, the response speed was unexpectedly accelerated, which is ascribed to the rapid and direct tunneling of electrons at the M/S interface as well as to the reduction in RC time constant based on the data analysis and the underlying physical principle discussion. The treatment conditions can be further optimized to counterbalance some side effects. These findings reveal an efficient technique for comprehensively improving the performance of β-Ga2O3 solar-blind PDs.