Ocean sediments are considered to contain microbial biomass that equals the stock of organic matter on all the continents combined. Knowledge on the spatiotemporal distribution and abundance of microbial life in marine subsurface sediments, however, is still sparse. A region particularly understudied in this respect is the Antarctic continental margin, in which the deep biosphere is largely terra incognita. A 794 m-long sediment sequence (Site U1532), recovered during IODP Expedition 379: “Amundsen Sea West Antarctic Ice Sheet History” provides the unique opportunity to study the composition and abundance of the deep biosphere in polar regions of the Southern Hemisphere. Porewater profiles of sulfate and methane concentrations indicate that the sulfate-methane transition zone (SMTZ) is located at a depth of ~660 mbsf, making it one of the deepest SMTZ ever encountered. Stable carbon isotope measurements attest to the biological origin of the methane and provide direct evidence for an active deep-dwelling microbial community. Cell abundances decline with depth by three orders of magnitudes but increase again within the SMTZ. Complementary biomarker analysis indicates that this change in cell abundances is associated with a shift in the microbial community to predominantly methanogens throughout the SMTZ. Our data thus provides first insights into the microbial diversity and abundance of the deep biosphere in the yet largely unstudied marine subsurface sediments surrounding Antarctica. Combining our results with previous data of cell abundances in marine sediments suggests that current projections of microbial biomass appears to be overestimated and need to be downsized.