Electron-plasmon coupling, in which carriers are dressed by collective electronic oscillations, represents an important many-body interaction in solids. Here, using angle-resolved photoemission spectroscopy, we report the tunable electron-plasmon coupling in the layered transition metal dichalcogenide system ${\mathrm{ZrSe}}_{2-x}{\mathrm{Te}}_x$. The coupling manifests itself as nondispersive satellite features below the conduction-band minimum at the $\overline{M}$ point, with the energy scale significantly exceeding the longitudinal optical phonon energy in this case. We demonstrate that the plasmon energy can be tuned through two synergistic routes. First, the chemical substitution of Te modifies the band structure and dielectric constant, leading to a twofold increase in energy. In contrast, in situ alkali-metal deposition varies the carrier concentration, affecting both the effective mass and plasmon energy. By comparing the plasmon energy with the carrier concentration and effective mass, our results provide compelling evidence for the electron-plasmon coupling in this system. This work establishes ${\mathrm{ZrSe}}_{2-x}{\mathrm{Te}}_x$ as a versatile platform for engineering collective electronic excitations and offers intriguing means for manipulating many-body interactions in layered quantum materials.

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• Plasma oscillations
• Angle-resolved photoemission spectroscopy
• Density functional approximations