Microplastic (MP) pollution is a growing threat to West African coastal ecosystems, but Mauritania has no quantitative data to date. We present the first characterization of MP contamination in Nouakchott's coastal surface waters, sampling three sites with contrasting anthropogenic pressures Lwerf (port area), Hotel Sabah (urban tourism zone), and Fish Market (artisanal fishing hub) during the warm and cold seasons. In total, 237 particles were collected using plankton nets (20 µm mesh), examined under a stereomicroscope, and classified by size (small MPs, large MPs, mesoplastics, macroplastics), morphology (filament, fiber, film, fragment), shape, color, and length (0.1–125 mm). Non-parametric tests (Kruskal-Wallis, H = 54.16 – 67.95, p < 0.001; Mann-Whitney U, r = 0.535), chi-square/Fisher exact tests with Cramér's V (0.119–0.525), Spearman correlations, PERMANOVA (R²site = 0.400; R²season = 0.399; combined = 79.9%), PCA, and correspondence analysis (CA) were applied. Together, site and season accounted for roughly 80% of multivariate variance in MP composition. Season had the strongest effect on particle size (η²H = 0.282): cold-season samples were dominated by mesoplastics and macroplastics, while the warm season brought smaller particles. CA uncovered site-specific morphological signatures fibers at Hotel Sabah (morphologically compatible with domestic effluents), films at Fish Market (compatible with single-use packaging), filaments at Lwerf (compatible with fishing-gear materials). Shannon color diversity confirmed Lwerf as the most heterogeneous site (H' = 1.814, 95% CI [1.36–1.99]), pointing to contamination from multiple sources. These results establish a first morphology-based baseline for MP monitoring in Mauritania and call for management strategies tailored to each site and season. Source attributions are presented as hypotheses requiring polymer-level (FTIR/Raman) verification in future work.