The injury causes resistance in human gastric cancer (GC). This Adverse Outcome Pathway (AOP) entitled “Increases in cellular reactive oxygen species (ROS) and chronic ROS leading to human treatment-resistant gastric cancer (GC)” consists of molecular initiating event (MIE) as “Increases in cellular ROS” and “Chronic ROS”, followed by series of key events (KEs); “porcupine-induced Wnt secretion and Wnt signaling activation”, “beta-catenin activation”, “epithelial-mesenchymal transition (EMT), and adverse outcome (AO) as “human treatment-resistant GC” in the sequence. ROS has multiple roles in disease such as development and progression of cancer, or apoptotic induction causing anti-tumor effects. In this AOP, we focus on the role of sustained levels of chronic ROS to induce the therapy-resistance in human GC. EMT, which is cellular phenotypic change from epithelial to mesenchymal-like features, demonstrates cancer stem cell (CSC)-like characteristics in human GC. EMT is induced by Wnt/beta-catenin signaling, providing the rationale to have Wnt secretion and beta-catenin activation as KEs in the AOP.