Free radicals play a significant role in various detrimental biological processes, including protein denaturation and lipid peroxidation, contributing to the development of numerous human diseases. Hence, investigating the antiradical potential of the synthesized compounds becomes crucial to determine whether they can mitigate the levels of free radicals and provide protection against oxidative stress in the human body. The aim of the present investigation is the synthesis, characterization, and study of antioxidant activity of the new ligand: N-[4-({2-[1-(pyridin-2-yl)ethylidene]hydrazinecarbothioyl}amino)phenyl]acetamide (HL) and Cu(II) coordination compounds with HL: [Cu(L)CH₃COO] (1), [{Cu(L)Cl}₂]·H₂O (2), [Cu(L)H₂O·DMF]NO₃ (3). The new thiosemicarbazone based on 4-aminoacetanilide was synthesized, and functionalized according to the organic synthesis procedure: synthesis of the isothiocyanate group and hydrazone following the nucleophilic addition reaction with the formation of the HL ligand. The HL was characterized by NMR, FTIR spectroscopy, and X-ray crystallography. Single crystal X-ray diffraction analysis elucidated the structures of thiosemicarbazone HL as well as complexes 1-3. The antiradical activity against the 2,2-azinobis-(3-ethylbenzothiazoline-6-sulphonate) radical cation (ABTS^•+) was evaluated for the compounds under examination: HL and copper(II) complexes 1–3. The obtained results are represented as semimaximal inhibitory concentrations (IC₅₀). The HL and complexes 1-3 possessed notable antioxidant activity with IC₅₀ values of 8.5±1.5, 47.44±1.9, 24.3±1.3, 23.3±0.9 µM, respectively. Thus, the ligand HL and complexes 2, 3 exhibit remarkable antioxidant activity, surpassing the activity of Trolox, a standard antioxidant utilized in medical applications. The tested ligand HL demonstrates an activity that is four times higher than that of Trolox. Among the copper(II) complexes, the antioxidant potency follows this sequence: Cu(L)H₂O·DMF]NO₃ ≥ [{Cu(L)Cl}₂]·H₂O ≥ [Cu(L)CH₃COO].