In nature, lignin is a light-colored compound with many fascinating properties, such as the absorption of ultraviolet (UV) light, which can be utilized in various applications, including cosmetics, composites, and coatings. Although the UV-absorbing properties of lignin are usually preserved when it is isolated from its natural source, its light color is generally lost due to an increase in the degree of condensation and the formation of chromophoric groups. Here, we introduce a novel thioacidolysis-based pulping method with thiolactic acid and choline chloride, which allows for the extraction of white-colored lignin from softwood with both a whiteness index and ISO brightness over 90%, with a high yield (70% of total lignin). It is postulated that the thioacidolysis-type reaction between lignin and thiolactic acid results in efficient dissolution of lignin and diminishes its discoloration. Furthermore, the nanometric-sized morphology of lignin contributes to its white color by causing the diffusion reflectance of visible light. The high visible light reflectance of white lignin results in significantly lower heating of the model lignin surface under sun simulation compared to kraft lignin. Thus, with good emissivity at the atmospheric window, white lignin exhibits high potential as a UV-absorbing compound for passive radiative cooling materials.