Microplastics (MPs) — plastic particles smaller than 5 mm — have emerged as a ubiquitous environmental contaminant of global concern, with freshwater systems increasingly recognised as critical conduits for MP transport from terrestrial sources to marine environments. Urban rivers traversing densely populated metropolitan areas accumulate particularly high MP loadings from combined sewer overflows, stormwater runoff, wastewater treatment effluent, and direct littering, yet systematic multi-matrix characterisation of MP contamination in Indian urban rivers remains limited relative to the scale of the problem. India generates approximately 9.46 million tonnes of solid plastic waste annually, a disproportionate fraction of which enters river systems through inadequate solid waste management infrastructure.This study presents the first comprehensive multi-matrix MP characterisation across four major urban rivers in Tamil Nadu and Kerala — the Cooum and Adyar rivers in Chennai Metropolitan Area, the Periyar river in Ernakulam district, and the Pamba river in Pathanamthitta district — encompassing 18 sampling stations, four sampling seasons, and four environmental matrices (surface water, bed sediment, fish gastrointestinal tracts, and atmospheric bulk deposition). µ-FTIR spectroscopy identifies polymer types, stereomicroscopy characterises particle size, shape, and colour, and SEM-EDX examines surface morphology and elemental composition. Hazard Quotient (HQ) and Polymer Risk Index (PRI) assess ecological risk. The Cooum river exhibited the highest MP abundance (2,341 ± 312 items/m³ in surface water; 6,218 ± 634 items/kg dry weight in sediment), with polyethylene (32.4%) and polypropylene (24.7%) as dominant polymer types across all matrices. Significantly elevated MP concentrations were recorded in fish GIT (3.2 ± 1.1 MPs per individual), with 87.4% of sampled fish containing detectable MPs.