Micro- and nanoplastics have emerged as biologically active contaminants that connect environmental pollution with ecosystem dysfunction, microbial imbalance, animal toxicity, and human health uncertainty. This One Health review critically synthesizes current evidence on the sources, physicochemical characteristics, environmental fate, biological toxicity, microbial interactions, and medical relevance of micro- and nanoplastics in biological systems. These particles originate from primary manufactured materials and secondary fragmentation of plastic waste, synthetic textiles, agricultural plastics, tyre wear, packaging, wastewater, and atmospheric deposition. Their toxicity is strongly influenced by particle size, shape, polymer type, surface chemistry, aging, biofilm formation, and contaminant adsorption. In plants and ecosystems, micro- and nanoplastics can impair seed germination, root architecture, photosynthesis, nutrient cycling, soil microbial activity, crop productivity, and ecological stability. Animal studies further demonstrate oxidative stress, inflammation, neurotoxicity, reproductive injury, developmental disruption, gut damage, and metabolic disturbance, although many experiments still rely on high doses and pristine laboratory particles. Microplastics also create plastisphere biofilms that alter soil, aquatic, and gut microbiomes and may facilitate pathogen survival and antimicrobial resistance gene dissemination. Human exposure occurs through food, drinking water, air, dust, and consumer products, with increasing detection in blood, lung, placenta, breast milk, vascular tissues, and other biological samples. However, causal links with chronic disease remain uncertain due to limitations in detection, dose standardization, contamination control, and long-term epidemiological evidence. Overall, micro- and nanoplastics represent an emerging cross-sectoral biological threat requiring standardized monitoring, realistic toxicology, biosensor development, biotechnology-based remediation, safer material design, and integrated One Health risk assessment.