The influence of hydrological and hydrochemical conditions on the spatial distribution of microplastics in estuarine zones has been poorly studied. The study aims to assess microplastics spatial distribution and their quantitative and qualitative characteristics in the Lower Volga–Northern Caspian system. The data were collected during an expedition on R/V Deneb in September 2023. The samples were taken using a neuston trawl net (0.3 mm mesh) in the surface layer (0–0.2 m), followed by processing according to a modified NOAA method and direct visual sorting under a microscope. The polymers were identified using the Raman and Fourier-transform infrared spectroscopy, with the PHI hazard index calculated. In parallel, hydrochemical, hydrological and meteorological parameters of the environment were measured, including ion composition. In the Lower Volga, the average particle concentration was 81 particles/m³, with peak values of 100 particles/m³ (following a downpour in Kalmykia), as well as 92 particles/m³ near Volgograd and 98 particles/m³ near Astrakhan, indicating anthropogenic runoff as the primary source. In the Northern Caspian Sea, particle concentrations were significantly lower (ranging from 3 to 44 particles/m³, with an average of 13 particles/m³), showing a pronounced downward gradient from the estuary. Fine microplastics (0.3–2 mm) in the form of fibres and fragments predominated, mainly consisting of polyethylene, polyethylene terephthalate and polyamide, which are classified as hazard class 2 according to the PHI index. The spatial distribution of particles correlates with variations in salinity and ionic composition (a 20- and 80-fold decrease in the Ca²⁺/Cl⁻ and HCO₃⁻/Cl⁻ ratios), turbulent mixing following a storm and shower precipitation. In the estuary zone, the dilution of river water with seawater leads to a significant reduction in particle count: from 98 to 44 particles/m³, and subsequently to 13 particles/m³. The primary source of microplastics is anthropogenic runoff from the Volga, including diffuse runoff from urbanised areas. The specific nature of the Caspian Sea as a closed basin with a significant river runoff results in relatively higher microplastics concentrations than those found in open seas. The obtained data justify the need to control surface runoff and monitor estuarine geochemical barriers.