Novel series of heterocycliccompounds pairing hybrids pyrazole, naphthalene and pyrazoline/thiazoldinemoieties were synthesized. The starting compounds pyrazolyl hydrazinecarbothioamidewere synthesized by the reaction of the 4‐formyl pyrazole derivatives with thiosemicarbazide to afford the substituted carbothioamide in good yields. The carbothioamideswere used as starting materials for synthesis of a variety of pyrazolylhydrazinylthiazole derivativesandpyrazolylhydrazonothiazolidin derivatives. The structures of the newly synthesized compounds were deduced depending on their spectrum data and elemental analysis. The cytotoxicity of the selected compounds was screened in vitro against three human breast cancer cell lines. The four compounds showed a gradual decrease in cell viability and theonly compound thiazolidin‐4‐one10b showed IC5010.16, 8.25 and 4.88 μM against the above cell lines, respectively. Thus, it was selected for further in vitro biological investigations. The Structure‐activity relationship for these compounds was interpreted in terms of molecular docking.