In this paper, we propose a polar-based 8-PSK demapper for DVB-S2 receivers. This demapper computes the Log- Likelihood Ratio (LLR) of the transmitted bit using the polar coordinate of the received 8-PSK symbol. Thanks to interpolation and approximation, the resulting demapper outperforms the classical LLR receiver based on Cartesian coordinates. Simulation shows almost no degradation compared to the optimal LLR generation for a 5-bit input precision of the LDPC decoder.