Marine Benthic Group D (MBG-D) archaea, discovered by 16S rRNA gene survey decades ago, are ecologically important, yet understudied and uncultured sedimentary archaea. In this study, a comprehensive meta-analysis based on the 16S rRNA genes of MBG-D archaea showed that MBG-D archaea are one of the most frequently found archaeal lineages in global sediment with widespread distribution and high abundance, including 16 subgroups in total. Interestingly, some subgroups show significant segregations toward salinity and methane seeps. Co-occurrence analyses indicate significant non-random association of MBG-D archaea with Lokiarchaeota (in both saline and freshwater sediments) and Hadesarchaea, suggesting potential interactions among these archaeal groups. Meanwhile, based on four nearly complete metagenome-assembled genomes (MAGs) and corresponding metatranscriptomes reconstructed from mangrove and intertidal mudflat sediments, we provide insights on metabolic potentials and ecological functions of MBG-D archaea. MBG-D archaea appear to be capable of transporting and assimilating peptides and generating acetate and ethanol through fermentation. Metatranscriptomic analysis suggests high expression of genes for acetate and amino acid utilization and for peptidases, especially the M09B-type extracellular peptidase (collagenase) showing high expression levels in all four mangrove MAGs. Beyond heterotrophic central carbon metabolism, the MBG-D genomes include genes that might encode two autotrophic pathways: Wood–Ljundahl (WL) pathways using both H 4 MPT and H 4 folate as C 1 carriers, and an incomplete dicarboxylate/4-hydroxybutyrate cycle with alternative bypasses from pyruvate to malate/oxaloacetate during dicarboxylation. These findings reveal MBG-D archaea as an important ubiquitous benthic sedimentary archaeal group with specific mixotrophic metabolisms, so we proposed the name Thermoprofundales as a new Order within the Class Thermoplasmata. Globally, Thermoprofundales and other benthic archaea might synergistically transform benthic organic matter, possibly playing a vital role in sedimentary carbon cycle.