H 2 -driven reduction of hexavalent chromium (Cr(VI)) using precious-metal catalysts is promising, but its implementation in water treatment has been restricted by poor H 2 -transfer efficiency and high catalyst loss. We investigated the reduction of Cr(VI) through hydrogenation catalyzed by elemental-palladium nanoparticles (PdNPs) generated in-situ within biofilm of a membrane biofilm reactor (MBfR), creating a Pd-MBfR. Experiments were conducted using a Pd-MBfR and a non-Pd MBfR. The Pd-MBfR achieved Cr(VI) (1000 μg L -1 ) reduction of >99 % and reduced the concentration of total Cr to below 50 μg L -1 , much lower than the total Cr concentration in the non-Pd MBfR effluent (290 μg L -1 ). The Pd-MBfR also had a lower concentration of dissolved organic compounds compared to the non-Pd MBfR, which minimized the formation of soluble organo-Cr(III) complexes and promoted precipitation of Cr(OH) 3 . Solid-state characterizations documented deposition of Cr(OH) 3 as the product of Cr(VI) reduction in the Pd-MBfR. Metagenomic analyses revealed that the addition and reduction of Cr(VI) had minimal impact on the microbial community (dominated by Dechloromonas) and functional genes in the biofilm of the Pd-MBfR, since the PdNP-catalyzed reduction process was rapid. This study documented efficient Cr(VI) reduction and precipitation of Cr(OH) 3 by the Pd-MBfR technology.