Background: Trophoblast cell surface antigen 2 (TROP2) is a transmembrane glycoprotein which is associated with cancer growth. Overexpression of TROP2 in advanced gastric cancer (AGC) is known as a poor prognostic factor. However, the association between TROP2 expression and prognosis in patient with AGC treated with nivolumab (NIVO) is not known. Methods: Patients who received NIVO monotherapy as third- or later-line treatment (NIVO group) and those who received only fluoropyrimidine, platinum and taxanes (non-NIVO group) at our hospital between 2015 and 2019 were enrolled. High TROP2 expression was defined as ≥50% positive tumor cells with weak to moderate membranous staining or ≥10% tumor cells with strong membranous staining using immunohistochemistry. Overall survival from first-line chemotherapy (OS) and progression-free survival (PFS) from starting of NIVO were compared according to TROP2 expression and combined positivity score (CPS). Results: Of 111 patients, 61 were in the NIVO group and 50 were in the non-NIVO group. Patient characteristics in the NIVO/non-NIVO groups were median age 67 (range, 41-83)/64.5 (29-80) years; performance status 0, 20 (33%)/23 (46%); lymph node metastasis 41 (67%)/23 (46%); peritoneal metastasis 32 (53%)/26 (52%); liver metastasis 11 (18%)/6 (12%); lung metastasis 4 (7%)/4 (8%); TROP2 high 55 (90%)/43 (86%). Patient characteristics at starting of NIVO did not differ between TROP2 high and low in the NIVO group. Of 55 patients with TROP2 high, 22 patients (40%) were CPS≥1. In the non-NIVO group, OS from first-line chemotherapy did not differ between TROP2 high and low (median, 16.2 months [95%CI, 13.6-19.2] vs. 26.2 months [95%CI, 7.7-NA], HR 0.97, p=0.946). Meanwhile, in the NIVO group, patients with TROP2 high had significantly longer OS from first line chemotherapy than those with TROP2 low (median survival, 23.5 months [95%CI, 17.2-31.2] vs. 19.2 months [95%CI, 10.7-NA], HR 0.40, p=0.04). Progression-free survival (PFS) from starting of NIVO tended to be longer in TROP2 high group than those in TROP2 low group (median, 1.9 months [95%CI, 1.6-2.3] vs. 0.6 months [95%CI, 0.2-NA], HR 0.62, p=0.26). OS from first-line chemotherapy and PFS from starting of NIVO in patients with TROP2 high did not differ between CPS≥1 and CPS<1 (OS median, 24.0 months [95%CI, 13.8-39.7] vs. 23.2 months [95%CI, 17.0-32.4], HR 0.87, p=0.69; PFS median, 1.9 months [95%CI, 1.6-3.5] vs. 2.0 months [95%CI, 1.4-2.8], HR 0.90, p=0.72). Conclusions: Higher TROP2 expression related to longer survival in the patients with AGC treated with NIVO monotherapy.