Currently, open dumping remains the most common method of solid waste disposal in Thailand. However, most dumpsites are unsanitary, causing serious environmental pollution through leachate seepage into soil and groundwater. This study aimed to quantify and characterize microplastics (MPs) in leachate and soils at an open dumpsite, as well as to investigate the accumulation of associated toxins, specifically heavy metals and plasticizers. Heavy metals analyzed included Cu, Mn, Zn, Ni, Pb, Cd, Cr, Hg, and As, while plasticizers consisted of polycyclic aromatic hydrocarbons (PAHs), Bisphenol A (BPA), and phthalates. MPs were examined for abundance, morphology (shape, color), and polymer composition. The results showed that leachate quality parameters (DO, BOD, COD, EC, TS, and SS) at three sites before treatment and one site after treatment exceeded the industrial wastewater discharge standards set by the Ministry of Natural Resources and Environment (B.E. 2017). MPs contamination was highest in leachate around the dumpsite, followed by fresh leachate, untreated leachate, and treated leachate. MPs in soils within the dumpsite were also higher than in sediments. Morphological classification revealed five major forms: fragments (27%), fibers (40%), pellets (21%), films (7%), and flakes (8%). MPs occurred in seven colors: transparent, red, blue, black, pink, white, brown, and green. FTIR analysis identified polymers including polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polyester (PL), and chlorosulfonated polyethylene (CSPE). Heavy metals detected in leachate included Ni (1.52 ± 0.40 mg/L), Cd (2.62 ± 0.02 mg/L), Cr (0.97 ± 0.70 mg/L), and Pb (0.97 ± 0.52 mg/L), with significant differences across sites (p < 0.01), all exceeding the Pollution Control Department’s water quality standards. Plasticizers detected included dibutyl phthalate (DBP), di(2-ethylhexyl) phthalate (DEHP), butyl benzyl phthalate (BBP), PAHs (acenaphthene, acenaphthylene, anthracene, pyrene), and Bisphenol A. These substances are known to be harmful to human health and ecosystems, with some classified as carcinogens. The findings demonstrate that open dumpsites are significant sources of MPs and associated toxic contaminants in leachate and soil, with potential risks to food chains and ecological systems. Effective waste separation, monitoring, and management are urgently needed to mitigate microplastic pollution and toxic chemical contamination in Thailand.