Transient receptor potential canonical 6 (TRPC6) proteins form receptor-operated Ca²⁺-permeable channels, which have been thought to bring benefit to the treatment of diseases, including cancer. However, selective antagonists for TRPC channels are rare and none of them has been tested against gastric cancer. Compound 14a and analogs were synthesized by chemical elaboration of previously reported TRPC3/6/7 agonist 4o. 14a had very weak agonist activity at TRPC6 expressed in HEK293 cells but exhibited strong inhibition on both 4o-mediated and receptor-operated activation of TRPC6 with an IC₅₀ of about 1 μM. When applied to the culture media, 14a suppressed proliferation of AGS and MKN45 cells with IC₅₀ values of 17.1 ± 0.3 and 18.5 ± 1.0 μM, respectively, and inhibited tube formation and migration of cultured human endothelial cells. This anti-tumor effect on gastric cancer was further verified in xenograft models using nude mice. This study has found a new tool compound which shows excellent therapeutic potential against human gastric cancer most likely through targeting TRPC6 channels.