Hydrophobically modified ethoxylated urethanes (HEURs) comprising hydrophilic poly(ethylene glycol) backbone and hydrophobic end groups are widely applied as rheological additives to regulate the viscoelastic behavior of waterborne systems such as coatings and inks. However, the conventional HEURs are usually nonreactive and remain free in final films, which may bring some defects and damages to the final cured films during the service time. An appealing way to improve filming properties may be to design reactive HEURs, which can participate in the post-crosslinking reactions with other components in formula during the film curing. In this work, a polymerizable acrylic-type HEUR (acrylic-HEUR) end-functionalized by the reactive methacrylate group and hydrophobic octadecyl tail is reported. The typical free radical polymerization feature of acrylic-HEUR based on the highly reactive methacrylate group is carefully clarified by dynamic light scattering, in situ nuclear magnetic resonance (H-1 NMR), and rheological measurements. The excellent thickening performance of acrylic-HEUR in waterborne UV emulsions and its effective copolymerization reactivity with the UV-curable matrix resin are demonstrated by in situ rheological measurements under UV irradiation in detail. Compared to conventional nonreactive HEURs, the copolymerization reactivity of acrylic-HEUR may make it especially suitable for applications in waterborne UV coatings to avoid its migration from final UV-cured films. This work may not only open a new perspective for the fabrication of HEUR polymers with high chemical reactivity but also promise the potential of polymerizable HEURs as superior associative thickeners in industry.