The crucial feature of organoselenium compounds, when considering them as promising drug candidates in cancer therapy, is their unique ability to alter the cellular redox regulations. Organic Se-molecules continue to demonstrate a positive therapeutic effect both in cancer prevention—as antioxidants, and treatment—as prooxidants. The growing interest in this field of research highlights the need to search for particular pharmacophore motifs, which could enhance the efficiency and selectivity, and decrease the toxicity of potential anticancer agents. Herein, a series of redox-active organoselenium derivatives—N-functionalized benzisoselenazol-3(2H)-thiones, has been designed and synthetized. A new synthetic pathway, with the application of Lawesson’s reagent, has been developed and efficiently applied. The key steps involving microwave irradiation facilitated performing the reaction in solvent-free conditions, shortening the reaction time and significantly improving the overall yield of the process. Six N-alkyl derivatives have been obtained and tested as antioxidant catalysts and anti-proliferative agents. The N-propyl benzisoselenazol-3(2H)-thione was the best peroxide scavenger and the N-cyclohexyl derivative exhibited the best cytotoxic activity towards prostate cancer cell line DU145.