<p>Microplastic (MP) contamination has emerged as a pervasive environmental issue globally, yet information from high-altitude Himalayan freshwater ecosystems remains scarce. This study combines spatial distribution analysis with morphological characterization (SEM–EDS) to assess polymer weathering of microplastics in surface waters and the freshwater fish <i>Tor putitora</i> (Golden Mahseer) from three Himalayan systems of Uttarakhand, India—Bhimtal Lake, the Ladhiya River (Chalthi), and Pancheshwar (Saryu–Kali confluence). Surface water (100 L/site) and fish (<i>n</i> = 30/site) samples were processed following standardized digestion and filtration protocols. Representative MP particles from water and fish tissues were characterized using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy–energy-dispersive spectroscopy (SEM–EDS). Microplastics were detected in all samples, ranging from 2.4 ± 0.8 particles L⁻¹ in water to 3.2 ± 1.1 particles g⁻¹ in fish tissues. Fish from Bhimtal Lake exhibited the highest loads (6.8 ± 0.4 items/individual). Fibers dominated (74–82%), primarily black and blue, with most particles &lt; 500&#xa0;µm. FTIR analysis identified five major polymer types—nylon, polyethylene, polypropylene, polyethylene terephthalate, and polystyrene while SEM–EDS revealed surface cracking, pitting, and elevated O and Si, indicating oxidative degradation and inorganic adsorption. The integrated spatial–morphological approach demonstrates polymer weathering signatures and trophic transfer pathways in Himalayan rivers, identifying <i>T. putitora</i> as a sentinel species for inland plastic pollution. These findings provide a mechanistic understanding of microplastic transformation and highlight the urgent need for watershed-scale interventions to mitigate plastic contamination in high-altitude freshwater ecosystems. Spatial variation across sites was linked with polymer-specific surface degradation patterns to infer environmental transformation processes.</p>