A new series of phthalazine derivatives was synthesized from the reaction of 1‐chloro‐4‐(4‐phenoxyphenyl)phthalazine as a reactive starting material with different carbon, nitrogen, oxygen, and sulfur nucleophiles. The structural formulae of all products were confirmed and characterized by elemental analyses and spectral data (IR, 1H NMR, 13C NMR, and mass spectra). Some of the synthesized derivatives were screened for their antitumor activity against four human tumor cell lines using MTT assay. In comparison with doxorubicin as standard drug, compounds 1, 20, and 25 showed the most potent cytotoxic effect. In addition, investigation of antioxidant activity revealed that hydrazinylphthalazine 20 has the highest activity.