Subgroup analysis by cancer type demonstrated that high HHLA2 expression was associated with poor OS in patients with clear cell renal cell carcinoma (HR = 3.42, 95% CI: 2.39–4.91), gastric cancer (HR = 2.03, 95% CI: 1.31–3.16), intrahepatic cholangiocarcinoma (HR = 1.77, 95% CI: 1.24–2.53), lung cancer (HR = 2.14, 95% CI: 1.33–3.44) and other cancer types (HR = 2.08, 95% CI: 1.34–3.24), but not in patients with epithelial ovarian cancer (HR = 0.52, 95% CI: 0.08–3.56). Here, HHLA2 is linked to gastric cancer.