Molecular-level characterization of dissolved organic matter (DOM) in different urban water resources (river water, groundwater, reclaimed water, and rainwater) was performed by Orbitrap Fourier transform mass spectrometry (FT-MS). The mass spectra, which were fingerprints of DOM composition, were clearly different among samples. Based on accurate molecular mass determination, about 300 molecular formulae were identified. Most of the DOM molecules were composed of carbon, hydrogen and oxygen. Molecular formulae in river water consisted of CHO mostly and much less N- and S-containing compounds than other water samples. The percentage of CHO molecules was the highest for reclaimed water. The reclaimed water sample was characterized by a few peaks with strong intensities, which were presumed to be linear alkylbenzene sulfonates, their co-products and their biodegradation products based on their molecular formulae. Some of these compounds were also detected from river water, probably indicating the impact of wastewater treatment plants located upstream of the sampling point. Orbitrap FT-MS analysis is a powerful tool to discriminate DOM composition of urban water resources.