Nowadays, there is an increased awareness on the threat that marine litter may pose to the marine environment. This review describes the major concerns related to plastic pollution, namely in terms of toxicity of different types and sizes of nanoplastics (particles smaller than 100 nm) to marine organisms, either producers or consumers. The available data show that nanoplastics may affect negatively organisms from different phyla with reported effects ranging from alterations in reproduction to lethality. Nevertheless, no information regarding marine vertebrates (e.g., fish) was found. Data show a high potential for bioaccumulation/biomagnification along marine food chains, since they can easily be retained inside organisms. The lack of standardized methodology for nanoplastics detection and the poor or inexistent legislation makes nanoplastics an environmental challenge.