Stromal fibroblasts perform important roles in cancer development and progression. Overexpression of Twist1, a basic helix-loop-helix transcription factor, is often associated with aggressive behavior in many tumors. In this study, we investigated Twist1 expression patterns in gastric stromal fibroblasts and cancer cells using a monoclonal Twist1 antibody after validating the effectiveness of four commercial Twist1-specific antibodies. Twist1 expression was more frequently observed in gastric cancer-associated fibroblasts (CAFs) than in other cancer cells but was otherwise rarely expressed in noncancerous tissue. In laser capture microdissection of stromal fibroblasts, Twist1 immunopositive fibroblasts exhibited significantly increased Twist1, fibroblast-specific protein 1, and CXCL14 mRNA expression. Furthermore, Twist1 mRNA expression showed a significant linear correlation with that of platelet-derived growth factor receptors β and α. We found that conditioned media from Twist1-expressing skin and lung fibroblasts significantly promote invasion of gastric cancer cells in vitro. In 195 gastric cancer samples, CAF Twist1 expression was associated with tumor size, invasion depth, and lymph node metastasis. Twist1 was also associated with poor prognosis in patients with gastric cancer, particularly in those with the diffuse type. In conclusion, CAFs in gastric cancer frequently have altered Twist1 expression, and increased Twist1 expression in fibroblasts contributes to the progression of cancer cells and poor patient survival.