Knowledge of sedimentary organic carbon (OC) in hadal trenches, the deepest ocean realm, is rudimentary. Here, we conducted a comprehensive analysis of total OC (TOC), stable and radio-carbon isotopes (δ13C and Δ14C), and biomarkers (e.g., n-alkanes, n-alkanols and n-fatty acids) in 12 sediment cores collected from hadal (trench axis) and non-hadal (abyssal plains and slopes) settings of the Kermadec Trench (Southwestern Pacific) and Atacama Trench (Southeastern Pacific) regions. Our results show that the TOC in the Atacama Trench region (0.86% ± 0.69%) is significantly higher than that of the Kermadec Trench region (0.29% ± 0.08%), likely related to different surface primary productivity. In both trench regions, the hadal sites are generally characterized by more negative δ13C, higher TOC/TN ratio, and similar or higher abundance ratio of terrigenous/marine biomarkers as compared to the non-hadal sites, suggesting the selective preservation of terrigenous biogenic/fossil OC at the hadal trench axis. The linear increase in 14C age with sediment depth in non-hadal cores reflects steady depositional conditions, whereas seven out of eight hadal sediment cores show 14C age reversals presumably due to occasional mass-transport deposits. Our results suggest (1) a strong heterogeneity in sedimentary OC characteristics between trenches and within each trench and (2) relatively high accumulation rates of terrigenous OC in both the Kermadec Trench (0.35 ± 0.04 g m−2 yr−1) and the Atacama Trench (1.4 ± 0.5 g m−2 yr−1). Thus, hadal trenches appear to represent an important environment for terrigenous carbon deposition in the deep ocean.