Microbial transformation of arsenic (As) plays a key role in As biogeochemical cycling and affects the mobility, bioavailability, and toxicity of As. This study aims to investigate the accumulation of As in marine sediments at different water depths in the East China Sea and reveal the abundance and diversity of the aioA, arrA, arsC, and arsM genes through quantitative real-time polymerase chain reaction (qPCR) and high-throughput sequencing. Results showed that the As content in sediments ranged from 5.53 mg kg−1 to 17.70 mg kg−1, which decreased with water depth. Abundant As biotransformation genes with low diversity were identified in these sediments, of which arsM and arrA were the most abundant. Significant positive correlation exists between the arsM and arrA gene abundance and between arsC and aioA, indicating the co-occurrence of the As biotransformation genes in microbes in marine sediments. Metagenomics analysis revealed that arsM gene was mainly distributed in Alphaproteobacteria, Solibacteres, Deltaproteobacteria, Clostridia, and Bacilli in these sediments. Among the sediment properties, total N, total S, C/N, and TOC were important factors that shaped the abundance profile of the genes involved in As transformation. This study provides a picture of As biotransformation genes in marine sediments from the East China Sea, which may affect As transformation and the ultimate fate of As in a marine environment. Display omitted •As content in marine sediments from the East China Sea decreased with water depth.•As biotransformation genes were analyzed by metagenomics in marine sediments.•arsM and arrA were abundant and co-existed in marine sediments.•α-, δ-Proteobacteria, Solibacteres, Clostridia, were main microbes carrying arsM.•Total N, S, C/N and TOC were important factors shaping the diversity of these genes. This study depicts the As distribution and its biotransformation genes in marine sediments from the East China Sea.