Microplastics (MPs) are now pervasive across the global ocean, yet their abundance, polymer composition, and size distribution in the high seas remain poorly constrained due to heterogeneous sampling strategies and analytical thresholds used in previous studies. These inconsistencies have limited basin‑scale comparisons and hindered the development of robust global transport models. Here we provide the first harmonized assessment of MPs in the world’s high seas using a standardized analytical framework capable of identifying particles from 1 μm to 5 mm. Between March 2020 and November 2022, seawater samples were collected at 100 locations by 14 Japanese commercial tanker vessels operating along major international shipping routes. Approximately 1 m³ of seawater was taken from hydrant systems at depths of 3–14 m and filtered onboard through 10 μm plankton nets. Samples were pretreated by hydrogen peroxide digestion and sodium iodide density separation. Polymer identification was conducted using automated micro‑FTIR imaging for particles ≥20 μm and micro‑Raman imaging for particles between 1 and 20 μm. MPs ≥20 μm ranged from 22 to 4,660 particles m⁻³, with a geometric mean of 260 particles m⁻³. Hotspots exceeding 1,000 particles m⁻³ were observed in the Kuroshio, California, and Kamchatka Currents, as well as the North Atlantic Gyre, reflecting strong heterogeneity driven by major circulation systems. The median particle diameter was ~60 μm, indicating that small MPs dominate suspended particles in the open ocean. Polyethylene, polypropylene, polyethylene terephthalate (PET), and polymethyl methacrylate (PMMA) were the most abundant polymers. PET occurred mainly as fibers, whereas PMMA showed notable enrichment in the Pacific Ocean, particularly within the Kuroshio Current, suggesting region‑specific terrestrial sources and transport pathways. Power‑law analysis was applied to estimate concentrations of MPs between 20 and 300 μm, offering a practical approach for harmonizing datasets produced with different size thresholds. This study establishes the first globally harmonized baseline describing MP abundance, size distribution, and polymer composition across the high seas. It also demonstrates a scalable monitoring framework using commercial tanker fleets, enabling sustained observations over vast ocean regions beyond the reach of conventional research cruises. These results provide essential constraints for numerical models of MP transport and fate and lay the foundation for coordinated international monitoring of oceanic plastic pollution. Ongoing analyses of MPs <20 μm will further advance understanding of the full size spectrum of microplastic contamination.