In this work, we demonstrate the phonon-assisted vibronic lasing of a Yb-doped sesquioxide Yb:LuScO3 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 Yb:LuScO3 laser, which represents the longest achievable wavelength in the Yb-doped sesquioxide lasers. Moreover, the Raman spectrum indicated that the Eg phonon mode with a frequency of 472cm-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.