In this study, a series of derivatives of thiazolyl-pyrazole hybrid structures were designed to search for new heterocyclic compound-based antitumor agents. The designed target structures were synthesized with easy, practical, and efficient procedures. The antiproliferative effect of the synthesized compounds against cancer cell lines A549, MCF-7, and HepG2 was evaluated regarding inhibition concentration and selectivity index against healthy cell line CCD-34Lu. The results overall showed that the compounds had high antiproliferation against cancer cells compared to the doxorubicin-positive control. In particular, compound 11 A549 (SI: 3.58) and HepG2 (SI: 12.36) had high selectivity in cancer cell lines, while compounds 10h and 10o had high selectivity (SI: 10.74 for both) in MCF-7 cancer cell lines. The calculated theoretical pharmacokinetic properties revealed that they could be suitable drug candidates. In addition, in vitro test results indicate a correlation between the structure-activity relationships of the compounds. The various molecular modifications of thiazolyl-pyrazole hybrid compounds are promising for developing new anticancer drug candidates.