<p>Petroleum hydrocarbons are traditional pollutants in coastal sedimentary environments, and the concentration of total petroleum hydrocarbons (TPH) has been used as a proxy to assess oil contamination in sediments. In this study, we investigated the distribution of TPH and <i>n</i>-alkanes (<i>n</i>-C₁₄—<i>n</i>-C₃₄) along two <i>ca.</i> 4 m sediment cores from the East China Sea (ECS) and further analyzed the <i>n</i>-alkane-based diagnostic parameters. The TPH concentrations (7.7–21.7 μg/g, with an outlier of 60.3 μg/g) indicated slight petroleum pollution at both sites. Among the hydrocarbons, higher abundances were observed for long-chain <i>n</i>-alkanes <i>n</i>-C₂₉, <i>n</i>-C₃₁ and <i>n</i>-C₃₃. The terrestrial <i>n</i>-alkanes accounted for 25%–33% of the total <i>n</i>-alkanes (<i>n</i>-C₁₄ to <i>n</i>-C₃₄) based on the Terr/Total ratio. Multiple <i>n</i>-alkane diagnostic parameters, such as Carbon Preference Index (CPI) and Terrestrial to Aquatic Ratio (TAR), were higher in sediments at the site close to the Yangtze River Estuary, suggesting an elevated contribution of terrestrial organic matter due to significant riverine input. This finding is in agreement with the higher C/N ratio (7.3 to 7.7) and more negative δ¹³C values of TOC (−23.1‰ to −22.5‰) at this site than the those of coastal site with increased marine organic matter impacted by upwellings (C/N: 6.3 to 6.6; δ¹³C-TOC: −22.1‰ to −21.8‰). In addition to the natural source of hydrocarbons, the lack of correlations between TOC and TPH indicates that other sources, such as anthropogenic discharge activities, may also contribute to the accumulation of petroleum hydrocarbons in coastal sediments. These results provided insight into the sources of petroleum hydrocarbons and <i>n</i>-alkanes in coastal sediments.</p>