In this work, we demonstrate the phonon-assisted vibronic lasing of a Yb-doped sesquioxide $\mathrm{Yb}\text{:}{\mathrm{LuScO}}_3$ crystal. The electron–phonon coupling process was analyzed and the Huang-Rhys factor S was calculated to be 0.75 associated with the fluorescence spectrum at room temperature. By a rational cavity design to suppress lasing below 1100 nm, a continuously spectral tunability from 1121 to 1136 nm was realized in a $\mathrm{Yb}\text{:}{\mathrm{LuScO}}_3$ laser, which represents the longest achievable wavelength in the Yb-doped sesquioxide lasers. Moreover, the Raman spectrum indicated that the ${\mathrm{E}}_{\mathrm{g}}$ phonon mode with a frequency of $472{\mathrm{cm}}^{-1}$ was mainly devoted to the phonon-assisted transition process. This work broadens the achievable laser spectrum of Yb-doped sesquioxide, and suggests that the multiphonon–electron coupling strategy should be universal for other laser materials.