A laser diode-side-pumped electro-optically Q-switched Er:YSGG laser that emits orthogonally polarized laser radiation at 2.79 mu m is demonstrated. This laser has a dual cavity structure to compensate for the thermal-induced birefringence effect; additionally, negative curvatures were ground at the crystal ends of Er:YSGG to compensate for the thermal lensing effect. The laser achieved a maximum repetition frequency of 20 Hz, with a pulse energy and pulse width of 71 mJ and 58 ns, respectively. The maximum pulse energy was 81 mJ, and the pulse width was 46 ns at 17 Hz, these conditions yielded a peak power of 1.76 MW, and an optical-to-optical efficiency of 1.27%.