Dysregulation of specific microRNAs (miRNAs) is found to play a vital role in carcinogenesis and progression of gastric cancer (GC). In the present study, we investigated the expression profiles of miRNAs in gastric cancer. Let-7b was found downregulated remarkably in gastric cancer tissues and was correlated with Helicobacter pylori infection, tumor stage, and lymphatic metastasis. Ectopic expression of let-7b suppressed the growth, migration, invasion, and tumorigenicity of GC cells, whereas let-7b knockdown promoted these phenotypes. Bioinformatic analysis predicted collagen triple helix repeat containing 1 (Cthrc1) as a direct target of let-7b. Luciferase assay showed that let-7b repressed the activity of Cthrc1 through binding its 3'UTR. Western blotting also confirmed that the protein levels of Cthrc1 were decreased by let-7b. Cthrc1 was significantly upregulated and reversely correlated with let-7b levels in GC. Co-expression of let-7b and Cthrc1 without its 3'UTR could rescue cell growth, migration, and invasion inhibited by let-7b. These results suggest that let-7b may directly target Cthrc1 and function as a tumor suppressor gene in GC.