The use of oxo-biodegradable plastics has become increasingly controversial due to insufficient evidence supporting their environmental benefits. In particular, their biodegradability in marine environments remains poorly understood, sparking a global concern that the rapid abiotic fragmentation of these plastics could harm marine ecosystems. Here, we assessed the marine biodegradability of polyethylene and polypropylene films containing a commercial oxo-biodegradable prooxidant and compared them to their untreated counterparts using biochemical oxygen demand tests. Our findings revealed that while the prooxidant enhanced oxidative degradation and fragmentation of the polymers, it did not improve their marine biodegradability. Neither the initial nor the UV-exposed samples, with or without the prooxidant, exhibited significant marine biodegradability. The accelerated oxidative breakdown of these so-called oxo-biodegradable polyolefins, coupled with their limited marine biodegradability, may worsen the ocean microplastic pollution. Therefore, the potential environmental risks of using prooxidants should be carefully considered to prevent unintended negative impacts on the environment.