<b><i>Background and Aim:</i></b> 5-Fluorouracil (5-FU) is an anticancer agent that induces intestinal mucositis, which causes diarrhea and dehydration. The NOD-like receptor family pyrin domain-containing 3 (NLRP3) inflammasome is responsible for inflammatory response activation via caspase-1 cleavage and subsequent interleukin-1β (IL-1β) and IL-18 activation and secretion. The objective of this study was to determine the role of the NLRP3 inflammasome in 5-FU-induced small intestinal mucositis. <b><i>Methods:</i></b> Small intestinal mucositis was induced in wild-type, NLRP3^–/–, and caspase-1^–/– mice by intraperitoneal injection of 5-FU. Some mice received intraperitoneal injection of a caspase-1 inhibitor, recombinant IL-1β or IL-18, or neutralizing antibody against IL-1β. <b><i>Results:</i></b> Mice treated with 5-FU developed small intestinal mucositis with diarrhea and body weight loss, characterized by a decrease in villus height and the villus height-to-crypt depth ratio. These histological changes peaked on day 3 and were accompanied by an increase in mRNA expression of NLRP3 and IL-1β and protein expression of cleaved caspase-1 and mature IL-1β. Mature IL-18 protein expression was not affected by 5-FU administration. NLRP3^–/– mice exhibited less severe 5-FU-induced mucositis, and this phenotype was mimicked by genetic depletion or pharmacological inhibition of caspase-1. Small intestinal mucositis was aggravated by exogenous IL-1β and neutralized by IL-1β antibody treatment. Administration of exogenous IL-18 or anti-IL-18 antibody did not affect any parameters associated with mucositis. NLRP3, cleaved caspase-1, and IL-1β were expressed by inflammatory cells (mainly macrophages) in the lamina propria and damaged epithelial cells. <b><i>Conclusions:</i></b> NLRP3 inflammasome activation may exacerbate 5-FU-induced small intestinal mucositis via IL-1β maturation.