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• The high pressure phase transitions of HfSe 2 were studied. • Pressure-induced semiconductor-metal transition appeared in the original P-3m1 phase. • The metallization was due to the overlap of valance and conduction bands with the contribution of Hf d and Se p orbitals. • A new metallic I 4 /mmm structure in HfSe 2 was found under high pressure. [Display omitted] In this study, we report the phase transitions and semiconductor-to-metal (S-M) transition of hafnium selenide (HfSe 2) under high pressure through experiments and theoretical calculations. The starting hexagonal structure transforms to an intermediate phase and I 4 /mmm structure above 10 GPa, and then converts into the I 4 /mmm structure completely above 47.2 GPa. Pressure-induced S-M transition at ~10 GPa was revealed by infrared (IR) reflectivity and resistance measurements. Theoretical calculations indicate that the pressure-induced metallization occurs in the initial hexagonal phase, which arises from the overlap of the valance and conduction bands with the contribution of the Hf d and Se p orbitals under high pressure. Upon pressure releases, the I 4 /mmm phase transforms into the initial hexagonal structure. This work provides new insights into the structures and electronic states of HfSe 2. [ABSTRACT FROM AUTHOR]