The transition toward a circular plastic economy and sustainable chemical industries requires the development and implementation of green methods of free radical polymerization. In this study, the high-speed radical polymerization of N,N-diethyl acrylamide was achieved in high-temperature water (150 °C, 0.5 MPa) without using potentially contaminating organic solvents. Precipitation polymerization was completed within 2 min. The molecular weight of the resulting polymer (27.7 × 103; dispersity of 1.78) was comparable to that of polymers obtained by typical polymerization at 70 °C and significantly exceeding those of polymers obtained by polymerization in high-temperature organic solvents. This implies that the main factor governing the polymerization rate is a kinetic effect due to hydrogen bonding between the carbonyl groups and water molecules; under these conditions, disproportionation termination was suppressed. The polymerization process developed herein using high-temperature water offers the benefits of a short reaction time and a low environmental impact, thus holding great promise for industrial applications.