Diarrheal illnesses have an enormous impact on military operations in the deployed and training environments. While bacteria and viruses are the usual causes of gastrointestinal disease outbreaks, 2 Joint Base San Antonio-Lackland, TX, training populations experienced an outbreak of diarrheal illness caused by the parasite Cyclospora cayetanensis in June and July 2018. Cases were identified from outpatient medical records and responses to patient questionnaires. A confirmed case was defined by diarrhea and laboratory confirmation, and patients without a positive lab were classified as suspected cases. In cluster 1, 46 suspected and 7 confirmed cases occurred among technical training students who reported symptom onset from 12 June to 21 June. In cluster 2, 18 suspected and 14 confirmed cases in basic military training trainees reported symptom onset from 29 June to 8 July. Numerous lessons from cluster 1 were applied to cluster 2. Crucial lessons learned during this cyclosporiasis outbreak included the importance of maintaining clinical suspicion for cyclosporiasis in persistent gastrointestinal illness and obtaining confirmatory laboratory testing for expedited diagnosis and treatment.