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.