In this work, we report the pressure-dependent electrical transport and structural properties of SnSe. In our experiments an electronic transition from a semiconducting to semimetallic state was observed at 12.6 GPa, followed by an orthorhombic to monoclinic structural transition. Hall effect measurements indicate that both the carrier concentration and mobility vary abnormally accompanied by the semimetallic electronic transition. First-principles band structure calculations confirm the semiconducting-semimetallic transition, and reveal that the semimetallic character of SnSe can be attributed to the enhanced coupling of Sn-5s, Sn-5p, and Se-3p orbitals under compression that results in the broadening of energy bands and subsequently the closure of the band gap. The pressure modulated variations of electrical transport and structural properties may provide an approach to improving the thermoelectric properties of SnSe.