<p>The introduction of FLT3 inhibitors for the treatment of acute myeloid leukemia has demonstrated their efficacy and broadened available treatment options. FLT3 inhibitors have been shown to be safer than conventional chemotherapy; however, numerous adverse effects remain uncharacterized. In this report, we describe the case of a patient who developed bloody stools associated with intestinal tumor formation during gilteritinib therapy. The patient underwent multiple chemotherapy regimens but experienced disease recurrence. Gilteritinib was initiated following confirmation of FLT3-ITD mutation positivity. During gilteritinib therapy, the patient developed bloody stools, and endoscopic examination revealed a tumor with an ulcer located in the sigmoid colon. The tumor resolved without discontinuation of gilteritinib therapy. Based on the clinical course and pathological findings, the event is considered to be gilteritinib-induced. Although gastrointestinal (GI) bleeding has been documented in gilteritinib clinical trials, no prior cases of tumor formation have been reported. Furthermore, only one case has been reported in which endoscopy was performed for GI bleeding in a patient receiving gilteritinib therapy. Thus, this case may provide new insights into drug-induced GI manifestations associated with gilteritinib.</p>