Abstract: Estuaries serve as critical interfaces where terrestrial materials undergo mixing, transformation, retention, and redistribution before entering the ocean. They also function as biogeochemical reactors in which dissolved organic matter (DOM) interacts strongly with heavy metals. Estuarine DOM is characterized by complex sources, heterogeneous composition, continuous molecular-weight distribution, and abundant reactive functional groups. Through processes including complexation, adsorption, flocculation, and redox reactions, DOM modulates the partitioning and transformation of heavy metals among dissolved fraction, colloidal fraction, particulate fraction, and sedimentary phase, thereby driving their migration across multi-medium interfaces. In recent years, advances in characterization techniques—such as ultraviolet-visible absorption spectroscopy, three-dimensional excitation-emission matrix fluorescence spectroscopy, size-exclusion chromatography, size-fractionation techniques, and high-resolution mass spectrometry—have significantly promoted research on the source apportionment, compositional identification, molecular structural characterization of estuarine DOM, and its interaction mechanisms with heavy metals. The quantity and properties of estuarine DOM are co-regulated by multi-source processes including riverine input, industrial wastewater discharge, autochthonous production by phytoplankton and microorganisms, and sediment release. Meanwhile, environmental factors such as salinity gradients, ionic strength, and pH can further modify the composition, structure, and heavy metal-binding capacity of DOM. The interaction mechanisms between DOM and heavy metals are closely linked to DOM characteristics (e.g., molecular weight, aromaticity, functional groups) and environmental conditions. This review systematically summarizes the sources, composition, and characterization methods of estuarine DOM, with emphasis on the interaction mechanisms between DOM and heavy metals, the key influencing factors, and the regulatory effects of DOM on heavy metals. On this basis, future research should focus on the integrated application of multi-scale characterization techniques, quantitative identification of reactive ligands, analysis of multi-phase coupling processes, and responses to extreme events, with the aim of providing scientific support for heavy metal pollution risk assessment, water quality criteria revision, and ecological environmental management in estuarine systems.