Weyl semimetals have been classified into type I and type II with respect to the geometry of their Fermi surfaces at the Weyl points. we propose another class of Weyl semimetal, whose unique Fermi surface contains two electron or two hole pockets touching at a multi-Weyl point, dubbed a type-III Weyl semimetal. Based on first-principles calculations, we first show that the quasi-one-dimensional compound ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ is a type-III Weyl semimetal with larger chiral charges. ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ can support fourfold helicoidal surface states with long Fermi arcs on the (001) surface. Angle-resolved photoemission spectroscopy measurements are in agreement with the gapless nature of ${\left({\text{TaSe}}_4\right)}_2\text{I}$ at room temperature and reveal its characteristic dispersion. In addition, our calculations show that external strain could induce transitions in ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ among the type-III, type-II, and type-I Weyl semimetals, accompanied with the Lifshitz transitions of the Fermi surfaces. Therefore, our work experimentally indicates ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ as a type-III Weyl semimetal and provides a promising platform to further investigate the physics of type-III Weyl fermions.

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• Electronic structure
• First-principles calculations
• Surface states
• Topological materials
• Topological phases of matter