In this study, β-diketone derivative new carboxamide Eu(III) and Tb(III) lanthanide complexes were synthesized and characterized by FT-IR, elemental analysis, UV–Vis, and molar conductivity techniques. There is no electrolytic conductivity in the compounds. Moreover, the photoluminescence properties and thermal behavior of the synthesized lanthanide complexes were investigated. Photoluminescence data revealed that the Tb(III) complex showed metal-based f-f transitions. The β-diketone derivative carboxamide ligand exhibited as antenna that transmitted the excited energy to the emission energy levels of the Tb(III) ion. These metal complexes were screened in two different common cancer cell lines and a human normal embryonic kidney cell line for 48 h. The results showed that these complexes have more antiproliferative activities in the colon cancer cell line compared to the lung cancer cell line.