This article reports the first application of coupled total organic carbon cavity ring-down spectroscopy (TOC-CRDS) for the analysis of the δ¹³C signature of dissolved organic carbon (DOC) in freshwater samples. DOC represents a major, dynamic component of the global carbon cycle. The export of DOC from soils into rivers and groundwaters may be highly climate sensitive, and much of this export may occur in ephemeral fluxes. Thus, a robust, simple and inexpensive method for the continuous determination of DOC concentration and quality is urgently needed. We detail recent advances made in the analysis of the δ¹³C signature of DOC using a TOC-CRDS system optimised for the analysis of DOC with natural abundances greater than 2.5 mg L⁻¹ with no sample pre-concentration required and sample volumes of 40 mL. Precision between replicated samples was comparable to conventional analysis by gas-source isotope ratio mass spectrometry, yielding δ¹³C values with standard deviations of ± 0.5 ‰ for DOC concentrations higher than 1.5 mg L⁻¹. The utility of this technique for the analysis of DOC in samples with a broad range of compositions and concentrations (2.5–25 mg L⁻¹ DOC) is demonstrated. Since DOC δ¹³C can be measured continuously, ca. 45 min per measurement, this method enables the online monitoring of DOC in river water, water intakes and treated waters, allowing changes in DOC fluxes to be monitored in real time.