A large pool of organic carbon resides in the world's oceans in the form of dissolved organic matter (DOM),. DOM is operationally defined as the fraction of organic matter that passes through a filter with a given pore size (which can range from less than 0.1 μm to 0.46 μm). This fraction has a longer oceanic residence time — and is generally less biodegradable — than particulate organic matter (POM)^,,,. Processes transforming DOM into POM are therefore crucial for our understanding of the cycling of organic material in the oceans^,,,. The aggregation of marine colloids, which constitute 10–40% of DOM,,, is thought to be an important step in the transformation of DOM into POM. It has been suggested that colloids, as well as transparent exopolymer particles and large aggregates (‘marine snow’) can be viewed as polymer gels^,,. Whether free DOM polymers can indeed spontaneously assemble to form polymer gels has, however, not yet been shown. Here we present experimental observations that demonstrate that marine polymer gels can assemble from free DOM polymers, and that their formation mechanism, physical characteristics and mineralization can be understood in terms of polymer gel theory^,,. The principles and methods of polymer gel physics thus have the potential to provide profound new insights into the processes controlling the exchange between the DOM and POM pools and the cycling of marine organic matter.