Microplastics (MPs) have been detected in various human tissues, including the liver, placenta, and blood. However, studies about MPs in the human locomotor system are limited. This study evaluated the presence of MPs in the synovium of 45 patients undergoing hip or knee arthroplasty using micro-Fourier transform infrared spectroscopy, scanning electron microscopy, and Raman microscopy and investigated their association with clinical indicators and local cellular responses. A total of 343 MPs of nine common types were identified, with a mean abundance of 5.24 ± 2.07 particles/g and ranging from 1.16 to 10.77 particles/g. Although there was no clear correlation between MP abundance and demographics, MP abundance was higher in hip samples than in knee samples. In addition, a potential association was observed between MP abundance and specific clinical diagnoses. Transcriptomic analysis revealed that a three-fold increase in MP abundance corresponded to enhanced local cellular stress responses, particularly heat shock protein reactions. Our findings demonstrate the presence of MPs in human joints and suggest that further studies are needed to explore the intricate associations between MPs and anatomical location, clinical diagnosis, and local cellular responses. While concerns about MPs are growing, there's still not enough data on human internal exposure levels or their potential health risks. Our study identified and analyzed a substantial quantity of MPs within synovial samples, thus offering initial evidence of plausible correlations between the MP abundance and anatomical location, clinical diagnoses and local cellular responses in human subjects. These findings urge researchers to further evaluate the MP exposure levels at different anatomical sites and to explore the human health implications at concentrations that reflect environmental exposure, so as to advance our comprehension of human-MP interactions. Display omitted •Microplastic abundance in human synovium was 5.24 ± 2.07 particles/g.•Nine types of polymers were identified.•Microplastic abundances were related to anatomical location and diagnosis.•Increased microplastic levels were related to enhanced cellular stress response.•Actual accumulated microplastics can affect human health.