BACKGROUND AND OBJECTIVE: An outbreak of diphtheria was identified in Malappuram district, North Kerala, India, from July 2016 onwards as a sudden increase in the number of throat swabs with suspected diphtheria received in the Microbiology Lab of Government Medical College Manjeri, Malappuram district. The objective of this study was to confirm the outbreak and identify the organisms and their antibiotic sensitivity patterns. MATERIALS AND METHODS: A total of 554 throat swabs were received from July 2016 to June 2017 from neighbouring areas of Malappuram district for culture and sensitivity. Elek gel precipitation test and polymerase chain reaction of the isolates were performed at the State Public Health Lab, Thiruvananthapuram. All isolates of Corynebacterium diphtheriae possess diphtheria toxin gene. Antibiotic sensitivity tests were performed. Repeat swabs were not included in the study. RESULTS: Of the 29 (5.23%) confirmed cases, 18 (62%) were male and 11 (38%) were female. Fourteen (48.3%) cases were between 6 and 10 years of age with nine (64%) males and five (36%) females. Seven (24.2%) were between 11 and 15 years of age with four (57%) males and three (43%) females. Three (10.3%) were between 16 and 20 years of age (all males). Two (6.9%) cases from 26 to 30 years and one (3.4%) each from 21 to 25 years, 31–35 years and 36–40 years. All isolates were Penicillin sensitive. CONCLUSIONS AND DISCUSSION: Our study identified an upsurge of diphtheria in Malappuram district, North Kerala, among older children and adolescents. Most of them are partially immunised and immunity to diphtheria declines over time. Adults and adolescents thus become susceptible later in life. This emphasises the need for booster vaccination above five years in the endemic areas and making antidiphtheritic serum available in all major hospitals of these regions.