Acute diarrhea is a frequent problem worldwide, mostly due to gastrointestinal infections or food poisoning. Boswellia serrata could be active in the treatment of acute diarrhea due to its anti-inflammatory, antispasmodic, and antimicrobial activity. In this randomized, double-blind, placebo-controlled clinical study, 49 adults with acute diarrhea were randomly allocated to receive 250 mg of a lecithin-based delivery form of Boswellia serrata (CASP) or placebo for 5 days. The time it took to become healthy with stoppage of diarrhea (primary end point) was significantly shorter in the intervention group (3.08 vs. 4.44 days: p-value < 0.0001). The probability of subjects treated with CASP to recover sooner was equal to 80.2%. A significantly lower number of stools was observed in the CASP group over time (β = −0.17, p-value < 0.0001). A significant difference was observed between the two groups for abdominal pain, nausea, and GAE (global assessment of efficacy). In conclusion, the lecithin-based delivery form of Boswellia serrata extract could be a useful addition to the treatment of acute diarrhea in adults. CASP is safe and reduces the time it takes to become healthy, the frequency of stools, the abdominal pain and nausea of subjects with acute diarrhea. Further studies are needed to confirm these promising results.