Abstract Marginal seas are important transitional zones for the delivery of terrestrial organic matter (TOM) from land to the open sea, and they play an important role in the carbon cycle. Tracing the source of sedimentary organic matter (SOM) deposited in marginal seas is fundamental to our understanding of the dispersal, degradation, migration, and conversion of organic matter. This paper presents high-resolution records of bulk organic matter and biomarker proxies from Core T08 that was recovered from the inner shelf of the East China Sea (ECS), and aims to identify the contributions of marine and terrestrial organic matter over the past 3725 yrs. Total organic carbon (TOC) values were low (0.50%) and showed no significant change between 3725 and 1800 yr BP (Period I), and increased continuously from 0.40% to 0.86% after 1800 yr BP (Period II: 1800–750 yr BP; Period III: 750 yr BP–present). The TMBR′ (ratio of terrestrial to marine biomarkers) and δ13CTOC (δ13C of TOC) values showed steady TOM contribution during Period I and higher TOM contribution driven by the increased Changjiang River (CR)-derived TOM under strong East Asian Summer Monsoon (EASM) and El Nino during Period II. During Period III, the increase in marine organic matter (MOM) contribution was indicated by the TMBR′, and this was caused by enhanced marine productivity related to intensified vertical mixture that was driven by the strengthened East Asian Winter Monsoon (EAWM). δ13CTOC shows a contrary trend to the TMBR′ during Period III, probably influenced by variations in the C3 vegetation type during this period. Spectral analysis of the TMBR′ series for the last 1200 yrs shows cycles with periods of 119, 75–85, and 54 yrs, confirming that climate-related events influenced the variation in SOM under the modulation of solar activity and solar irradiance at the centennial scale.