Up to now, no information is available on the role of redox mediators in anaerobic degradation of phenol without exogenous electron acceptors. The effects of mediators on anaerobic biodegradation of phenol by Shewanella sp. XB were investigated for the first time in this study. The results showed that the removal efficiencies of phenol were enhanced by 2.87-, 3.31-, and 2.89-fold under fermentation conditions in the presence of 0.20 mM anthraquinone-2-sulfonate (AQS), 0.05 mM anthraquinone-2,6-disulfonate (AQDS), and 0.05 mM riboflavin, respectively, in 3-day incubation. Further analysis found that phenol was fermented via volatile fatty acids (VFAs) as intermediates, particularly acetate, and further converted to CH4. Under denitrification conditions, phenol removal efficiencies were enhanced by 3.92-, 4.58-, and 3.57-fold when AQS, AQDS, and riboflavin, respectively, were added into reaction systems. During this process, phenol was preferentially degraded to CO2 and H2O rather than CH4. These results indicate that the three mediators not only increased significantly phenol biodegradation rates under fermentation conditions, but also considerably accelerated nitrate bioreduction-coupled phenol degradation. These findings are beneficial to develop efficient strategies of phenol biodegradation and to better understand phenol fate in natural environments.