Dissolved organic matter (DOM) cycling across the land-ocean continuum is highly complex, and our limited understanding of DOM molecular transformations hinders a full assessment of land-ocean connectivity in the global carbon cycle. Here, we applied one- and two-dimensional high-field ¹H nuclear magnetic resonance (NMR) spectroscopy and ultrahigh-resolution mass spectrometry (FT-ICR-MS) to investigate sources and transformations of solid-phase extractable DOM along an Amazonian mangrove-fringed river-to-ocean transect. Relative abundances of aromatic compounds decreased from the river to the coastal ocean, whereas aliphatic compounds increased. NMR spectroscopic features, commonly associated with carbohydrates, are probably related to flavonoid- and lignin-derived structural motifs. These structural features were more readily detected by ¹H NMR spectroscopy, whereas aromatics were more effectively detected by FT-ICR-MS. We tentatively identified polycyclic aromatic sulfur-containing compounds as being predominantly derived from urban areas, whereas sulfurized aliphatic compounds originated from sulfidic mangrove sediments. Surprisingly, while the number of DOM molecular formulas decreased along the river-to-coastal continuum, coastal marine DOM exhibited greater structural diversity than terrigenous DOM. Here, we showed that the interplay of distinct molecular pathways, particularly (photo)oxidation processes and sulfur incorporation, structurally diversifies DOM in coastal marine environments.