NPS-2143 is a calcium-sensing receptor (CaSR) antagonist that has been demonstrated to possess anticancer activity. To date, the effects of NPS-2143 on gastric cancer (GC) cell growth, motility, and apoptosis have not been investigated. In the present study, we firstly investigated the expression of CaSR in GC tissues using immunohistochemistry and western blotting. Then Cell Counting Kit-8 and colony formation assays were conducted to explore the effect of the NPS-2143 on the proliferation of GC cell line AGS. Transwell invasion and migration assays were performed to test the effect of NPS-2143 on AGS cell motility. We determined the percentage of apoptotic cells by flow cytometry and explored the changes of apoptosis-related protein by western blotting. Furthermore, we constructed a CaSR knockdown AGS cell line to determine whether NPS-2143 acted via inhibition of CaSR. We found that the protein expression level of CaSR was higher in GC tissues compared with the paired adjacent normal tissues. In addition, NPS-2143 treatment caused an inhibitory effect on the proliferation, invasion, and migration of AGS cells and a promoting effect on AGS apoptosis. The expression of Bcl-2 was decreased while the levels of Bax and active caspase 3 were enhanced in AGS cells after NPS-2143 treatment. Mechanistically, NPS-2143 lead to a significant decrease in the expression of phosphorylated (p)-AKT, phosphorylated mechanistic target of rapamycin (p-mTOR), p70, and cyclin D1. Knockdown of CaSR also suppressed cell proliferation, invasion, and migration and promoted cell apoptosis. No significant difference was observed between CaSR-silenced AGS cells with and without NPS-2143 treatment. These results confirmed that NPS-2143 has an inhibitory influence on AGS cell growth via inhibiting CaSR, which then suppresses the PI3K/Akt signaling pathway.