A new leaf spot disease was observed during the 2014 on Ocimum gratissimum in the BSIP Garden of Uttar Pradesh (India), causing significant damage to the host. The symptoms produced by the associated fungus consisted of necrotic dark brown, circular to sub-circular, up to 5 mm spots on both surfaces of the leaves. A fungus having distinct dictyospores (muriform) similar to those produced by Alternaria spp. was observed. The morphological characteristics of the fungus as well as the pathogenicity on the host confirmed that the pathogenic fungus is Alternaria alternata. To the best of our knowledge, Ocimum gratissimum, is here for the first time reported as a new host for the pathogenic fungus A. alternata causing leaf spot disease in Uttar Pradesh, India.