An outbreak of acute gastrointestinal illness occurred among personnel in an army camp in West Sikkim, India, between 27 June and 4 July 2023. The investigation aimed to determine the extent of the outbreak, identify the causative organism and source of infection, and implement effective control measures. A case was defined as any individual residing or working in the camp who, during the outbreak period, experienced ≥3 loose stools within 24 h, accompanied by abdominal pain or vomiting. Clinical samples including stool and rectal swabs, along with food and water samples, were collected and tested using conventional bacteriological culture methods and by multiplex PCR. Data were organized in Microsoft Excel to prepare a line list and epidemic curve was plotted by date and time of onset acute gastroenteritis (AGE) along with calculation of age specific attack rates. A total of 30 army personnel living in an army camp suffered from AGE with an attack rate of 13.5%. The mean age of case patients was 35.7 years (standard deviation ±6.85). Culture results revealed the presence of multiple bacteria including Escherichia coli and Klebsiella spp. The presence of Shiga toxin-producing E. coli (STEC) was confirmed by multiplex PCR in rectal swab samples. The findings suggest that the most probable cause of the outbreak was contamination of the in-camp water supply network due to inadequate maintenance and possible sewage infiltration. Timely clinical intervention and public health response effectively curtailed the outbreak and prevented further transmission.