In this study, hydrocarbon (HC) degradation activity of a HC-rich marine sediment was assessed in anaerobic microcosms during a 224 days incubation period. Natural TOC/N/P ratio of the sediment porewater (1,000/5/1) was gradually decreased to 1,000/40/6 which resulted in approximately ninefold increase in gas production (CH4+CO2) and HC removal. Addition of external HCs to the microcosms was also resulted in approximately twofold higher gas production and HC removal. A high proportion (92%) of aromatic HCs and all n-alkanes were removed from the microcosms under unlimited nutrient supply conditions without external HC addition. The microorganisms of the sediment degraded a wide range of aliphatic (n-C9-31 alkanes and acyclic isoprenoids) and aromatic (18 different one- to five-ring aromatics) HCs. Monitoring functional gene and transcript abundances revealed that methanogenesis and dissimilatory sulfate reduction took place simultaneously during the first 126 days, afterwards, only the syntrophic methanogenic consortium was active. Genes and transcripts related to initial activation of HCs were highly abundant throughout the incubation period showing that fumarate addition was the main pathway of anaerobic HC degradation. In conclusion, biostimulation of highly polluted anoxic marine sediments via nutrient amendment is effective and may constitute a suitable and cost-effective field-scale bioremediation strategy.