Cyclopolymerizable monomers (CPMs) are rarely described as cross-linkers, as their most prominent feature is the ability to form soluble polymer chains. In this study, we design a dimalonate-based CPM and investigate its ability as a low-shrinkage reactive diluent in a commercial resin. It demonstrates high reactivity in radical photopolymerization with significantly reduced volumetric shrinkage and shrinkage stress compared to commercial monomers. The investigated CPM is copolymerizable with conventional methacrylates and yields photopolymers with similar mechanical properties. Additionally, an in-depth evaluation of the propagation step during cyclopolymerization was performed via NMR studies. Considering the low volumetric shrinkage of the CPM-cross-linker, additive manufacturing or dental resins represent highly promising applications.