Onion crop in Rajasthan is severely affected by purple blotch, an important foliage disease. In Jhunjhunu district, this disease appears every year during Rabi but information on the exact pathogen responsible for the disease in this area is lacking. This work was conducted to determine the % disease index (PDI) of the disease, isolate and identify the pathogen and validate its pathogenicity. Survey of onion fields revealed that purple blotch was prevalent in all surveyed areas. The maximum PDI of 48% was recorded in Balwantpura village of Nawalgarh tehsil, at the same time the minimum PDI of 16.40% was observed in Majhau village of Udaipurwati tehsil. Diseased leaf samples showing typical purple blotch symptoms were collected. Fungal isolation was carried out on Potato Dextrose Agar (PDA). Both morphological examination and molecular analysis were used to identify pathogen. BLAST analysis of the ITS sequence revealed Alternaria tenuissima as the causal agent. Pathogenicity of the isolated culture was proved on healthy onion plants under controlled conditions. The findings of this study will help in developing proper management practices for the disease in this area.