We demonstrated an improved-beam-quality improved Er:YSGG laser emitting at wavelength of 2.79  μm by applying the Innoslab hybrid stable–unstable resonator and composite crystal with undoped YSGG section bonded to the pump side. With a space-combined, high peak power 976-nm quasicontinuous laser-diode stack and optimized coupling system, we obtained the maximum average output power of 10.2 W at 100 Hz repetition frequency and 500  μs pulse width, corresponding to the slope efficiency of 16.6%. The beam quality factors of M² were measured to be 3.3 and 2.6 in the fast and slow axis directions, respectively. To the best of our knowledge, this is the first mid-infrared laser designed with Innoslab hybrid stable–unstable resonator ever reported.