Signal transducers and activators of transcription 3 (STAT3) represent a transcription factor that is constitutively activated in various cancers. Numerous studies have shown that STAT3 plays crucial roles in cell proliferation and survival, angiogenesis, tumor-promoting inflammation, and suppression of antitumor host immune response in the tumor microenvironment. In this study, we investigated a novel inhibitor, called -6b, to target STAT3 in colorectal cancer cells. The influence of 5Br-6b on the proliferation of colorectal cell lines SW480 and HCT116 was evaluated using an 3-(4, 5-dimethylthiazolyl)-2 and 5-diphenyltetrazolium bromide assay. We detected cell apoptosis after the treatment of 5Br-6b by flow cytometry. In addition, 5Br-6b caused the cleavage of caspase-3 and decreased the expression of Bcl-2. Cancer cell invasion and migration were measured by transwell and wound-healing assay. The potential mechanism was evaluated by western blotting and immunofluorescence. The results show that 5Br-6b inhibits the activation of STAT3, and decreases the expression of its target genes that regulate cell proliferation, migration, and apoptosis. Thus, 5Br-6b is a promising therapeutic drug candidate for colorectal cancer by inhibiting persistent STAT3 signaling.