Microplastics (MPs) can enter soils via residues of agricultural mulching films, land application of sewage sludge, irrigation inputs, and atmospheric deposition, posing potential risks to soil biota and ecosystem functions. The toxic effects of MPs on earthworms have become a focal topic in soil ecotoxicology. This review synthesizes the toxic responses and key endpoints of earthworms to MP contamination at the individual, tissue, and molecular levels. It summarizes behavioral and fitness endpoints such as avoidance, growth, and reproduction along with mechanistic evidence including histopathology, oxidative stress, DNA damage, and gut microbiota dysbiosis. We compare the effects of different polymers (e.g., polyethylene, polylactic acid, and polystyrene) across particle size, shape, and aging states, and identify research gaps regarding environmentally relevant concentrations and across generations. Experimental and modeling approaches should be integrated to determine microplastic toxicity thresholds in earthworms, strengthening research on aged MPs and developing assessment frameworks that integrate traditional indicators with multi-omics approaches to support ecological risk assessment and management of soil MPs.