In this study, humic acid (HA) was extracted from peat soil and added to an arsenic-contaminated paddy soil for microcosm experiments to investigate its effect on microbial As(V) reduction in the anoxic paddy soil. The concentration of dissolved As(III) with amendment of HA increased gradually over time and, at the end of the incubation, reached 4.2 mg L−1, which is three times that of the control without amendment. Total RNAs were extracted to profile the potentially active microbial communities and suggested that HA increased the abundance o f Variovorax, Azoarcus, and Anaeromyxobacter sp. relative to the control. Transcriptions of the respiratory As(V)-reducing gene arrA (106 copies mL−1) were significantly higher than those of the cytoplasmic As(V)-reducing gene arsC (104 copies mL−1). The addition of HA increased the transcripts of arsC only during Day 2–6, but increased those of total bacteria and arrA during the whole time-course incubation. Our results indicated that the HA extracted from soil enhanced As(V) reduction primarily via up-regulated expression of respiratory As(V)-reducing gene.