In July 2014, a disease was observed in commercial fields and greenhouse crops of sweet basil (Ocimum basilicum) on ∼10 ha in Azapa Valley, Arica, Chile. Disease incidence ranged from 5 to 45% among fields and greenhouses. The pathogen infected stems, leaves, and inflorescences. Brown, necrotic lesions, each 0.5 to 2.0 cm in diameter, formed on the infected tissues. Lesions expanded rapidly under the humid and poor-aeration conditions of these fields and greenhouses, leading to total collapse of infected plants. On stems, leaves, and blossoms, the pathogen produced profuse conidia and mycelia, resulting in a mouldy gray appearance. B. cinerea was isolated consistently by incubating small pieces of infected leaves and stems sampled from the margin between diseased and healthy tissues. Based on morphological characteristics, results of molecular investigations (sequence analysis of the internal transcribed spacer region of ribosomal DNA and DNA-dependent RNA polymerase subunit II gene) and pathogenicity tests, the causal agent was confirmed as B. cinerea. This is thought to be the first report of gray mould in Azapa Valley in northern Chile.