As an oxidative hair dye, 2,5-diaminotoluene dihydrochloride is permitted up to a maximum concentration of 3.2% in Korea. An analytical method for percutaneous absorption of 2,5-diaminotoluene dihydrochloride was developed using liquid chromatography-tandem mass spectrometry (LC-MS/MS) with multiple reaction monitoring (MRM) and validated including Swab (SW), Stratum Corneum (SC), Skin (dermis + epidermis, SK), and Receptor Fluid (RF, 0.9% NaCl). The developed method demonstrated high linearity (r² = 0.9955–0.9997), accuracy (85.8%–112%), and precision (1.0%–10.3%) according to validation guidelines. The percutaneous absorption study was conducted using the Franz diffusion cell system, applying oxidative hair dye formulations containing 1% and 3.2% 2,5-diaminotoluene dihydrochloride dissolved in H₂O₂ at 10 mg/cm² to mini pig skin. Receptor fluid samples were collected at 0, 1, 2, 4, 8, 12, and 24 hours post-application. After 24 hours of application, the amount of 2,5-diaminotoluene dihydrochloride in each matrix was determined using LC-MS/MS. As a result, the total percutaneous absorption rates of 2,5-diaminotoluene dihydrochloride were determined to be 1.38 ± 0.99% for the 1% formulation and 1.10 ± 0.78% for the 3.2% formulation. These data can be applied to the exposure and risk assessment of 2,5-diaminotoluene dihydrochloride contained in hair dye products