Abstract: Recent epidemiological data have demonstrated a decrease in the overall incidence of gastric cancer. However, in young and middle-aged adults, the incidence of gastric cancer has not decreased; in fact, it has increased to some extent. The particular type of gastric cancer affecting young and middle-aged patients is referred to as early-onset gastric cancer (EOGC). To date, the precise age of onset for EOGC remains uncertain, but most studies have proposed an age cut-off of 40 years for its diagnosis. Compared with late-onset gastric cancer, EOGC is more commonly observed in female patients, and is associated with specific characteristics including low differentiation, diffuse type, and a high incidence of peritoneal metastasis. Common subtypes of EOGC include the Epstein-Barr virus (EBV)-positive type and the genomically stable type. Moreover, EOGC is commonly associated with mutations in the CDH1 and TGFBR1 genes. These factors contribute to a higher malignancy, and are unique clinical, pathological, and molecular characteristics of EOGC. To gain a better understanding of EOGC, this study aimed to summarize recent research findings on this disease, including its epidemiology, etiology, clinical features, and pathological characteristics.