<p>Micro- and nanoplastic particles are ubiquitous environmental pollutants, threatening human health, aquatic and soil ecosystems. These minute synthetic fragments, persisting for centuries, infiltrate the food chain, posing potential health risks through bioaccumulation in various tissues, toxicity and exposure to associated chemicals. Although macro- and microplastics are intensively examined in environmental and biological research, information on nanoplastics with diameters below 1 μm is limited. Such particles can cross biological borders, including the blood–brain barrier, posing a greater health risk than microplastics. Apart from the mere detection of such particles, gaining an understanding of size distribution, numbers and size limits will be crucial in assessing their impact on global ecosystems and human health. Here we establish an optical sieve that uses Mie void resonances for nanoplastic detection and sizing. The optical sieve consists of arrays of optically resonant voids with different diameters that simultaneously serve as filtering and sorting elements, as well as all-optical reporters, requiring only an optical microscope and a standard camera with an RGB sensor in combination with colorimetric analysis. The system is evaluated using a synthesized real-world sample with a plastic particle mass concentration of 150 μg ml⁻¹. Our approach consequently delivers statistical information on numbers, size and size distribution via the observation of distinct colour changes, overcoming the need for advanced techniques such as scanning electron microscopy. The proposed method offers a straightforward, highly accessible and mobile solution, making it an efficient and easily implemented tool for environmental and biological research.</p>