Background: Although FLOT (5-fluorouracil [5-FU]/leucovorin [l-LV] + oxaliplatin [L-OHP] + docetaxel [DTX]) is globally recognized as the standard treatment for gastric cancer (GC), the safety of FLOT in Japanese patients (pts) is unknown. Moreover, because advanced GC (AGC) pts with severe peritoneal metastasis (SPM) have often been excluded from pivotal clinical trials due to their poor condition, it is necessary to develop standardized treatment for them. Methods: This study is an open-label, multicenter, single-arm, phase I study to determine the recommended phase II dose (RP2D) of FLOT for Japanese AGC pts without (part 1) or with (part 2) SPM using a 3 + 3 design. Main eligibility criteria included histologically confirmed AGC, HER2 negative, and age 20–75 years. In part 2a, pts with ECOG Performance status (PS) 2, who simultaneously had massive ascites and inadequate oral intake, were excluded. Massive ascites was defined as continuous ascites from the pelvic cavity to the upper abdomen, based on computed tomography findings. Inadequate oral intake was defined as the requirement for daily intravenous infusions to supply water or nutrition. The dose limiting toxicity (DLT) was assessed before the start of cycle 2. The fixed dose of L-OHP, l-LV, and 5-FU was 85, 200, and 2600 mg/m², respectively, and DTX was administered at a dose of 50, 40, and 30 mg/m² in the level 0, −1, and −2, respectively. Results: In part 1a, one patient died of hemorrhage from the primary tumor on Cycle 1 Day 2 and was replaced. The characteristics were as follows: median age (range), 70 (51–71) years; male/female, 2/1; ECOG PS (0/1), 2/1; histological type (diffuse/intestinal), 3/0; prior gastrectomy (yes/no), 0/3; and measurable lesions (yes/no), 1/2. During DLT evaluation, grade 3 (G3) or higher adverse events (AEs) were neutropenia (n = 1), lymphocytopenia (n = 1), anemia (n = 1), and anorexia (n = 1). During follow-up, the most common G3 or higher AEs included neutropenia (n = 3), lymphocytopenia (n = 1), anemia (n = 1), febrile neutropenia (FN) (n = 1 at cycle 5), diarrhea (n = 1), anorexia (n = 1), fatigue (n = 1), and colonic perforation (n = 1) due to diverticulitis (not treatment-related AE). In part 2a, the characteristics were as follows: median age (range) 64 (49–69) years; male/female, 3/0; ECOG PS (0/1), 0/3; histological type (diffuse/intestinal), 3/0; prior gastrectomy (yes/no), 0/3; and measurable lesions (yes/no), 0/3. Two pts had massive ascites and 1 pt had both massive ascites and inadequate oral intake. During follow-up, only G3 neutropenia (n = 3) was observed. Two pts needed dose modification in cycle 2. Both in part 1a and 2a, no treatment-related deaths and no DLTs were observed and the RP2D was determined in level 0. Conclusions: We determined that DTX 50 mg/m², L-OHP 85 mg/m², 5-FU 2600 mg/m², and l-LV 200 mg/m² were the RP2D of FLOT for Japanese AGC with/without SPM. Clinical trial information: 041200044.