An enormous source of clean energy, called salinity gradient (SG) energy, exists from mixing waters with different salinities. Harvesting SG energy has attracted lots of attentions to develop efficient technologies. However, the power output is still limited. In this study, a concentration flow cell (CFC) powered by a bioelectrochemical system, defined as a bio-CFC, was proposed to recover SG energy from synthetic seawater (30 g/L NaCl) and river water (1 g/L NaCl) efficiently. The maximum power density of the bio-CFC reached 42 ± 2 W/m2, which was three times higher than that of a single CFC (10.6 ± 0.1 W/m2). The significant improvement was attributed to the additionally developed capacitive potential, which was formed by the bioelectrochemical currents from degradation of the organics in wastewater. The capacitive potential enhanced the Na+ intercalation/deintercalation on the electrodes and accelerated the Cl− transfer across the anion exchange membrane. This new strategy provides a promising way to recover energy from seawater and wastewater.