Riverine dissolved organic carbon (DOC) constitutes a pivotal component in the Earth's carbon cycle, yet little is known about the global patterns, sources, and factors governing lotic DOC. Here, we integrate a global dataset and employ machine learning to generate a global atlas of riverine DOC concentration and its radiocarbon (Delta C-14) and stable-carbon (delta C-13) isotopic signatures. Globally, riverine DOC has an average Delta C-14 value of -22.5 parts per thousand +/- 144.0 parts per thousand (radiocarbon age of 221 years), with fossil carbon contributing a minor fraction (6.7% +/- 3.0%). Terrestrial and autochthonous riverine production are the dominant DOC sources (>80%) at the global scale, with contemporary terrestrial DOC predominant in tropical rivers and within-river production prominent in those within temperate and semi-arid regions. Rivers draining high-latitude regions and high-elevation sites have the lowest Delta C-14 values (-353 parts per thousand to -78 parts per thousand; ages between 3400 and 600 years). River Delta C-14-DOC values correlate with soil organic carbon Delta C-14 values, but river DOC has much higher Delta C-14 values than subsurface soils indicating that riverine DOC originates from surface rather than subsurface soils. Because warming mobilizes aged organic carbon from permafrost soils, export to and processing of old carbon in recipient aquatic systems may accelerate with climate change.