As part of our ongoing studies on the synthesis of bioactive coumarin compounds, we synthesized a series of new coumarin‐fused 1,4‐thiazepines using a simple method. The biological activity of target compounds along with 3‐(2‐hydroxybenzylidene)chroman‐2,4‐dione intermediates was screened by evaluation of their antioxidant and cytotoxic activities. The antioxidant activity was assessed using two methods, namely, 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH) radical scavenging method and ferric reducing antioxidant power (FRAP) assay. 4‐Methoxyphenolic compound 5d in thiazepine series showed the most potent scavenging activity, while the 4‐bromophenolic derivative 5b was the most efficient compound in FRAP assay. Also, the result of cytotoxic evaluation using MTT [3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyl tetrazolium bromide] assay demonstrated that compound 5b is at least twofold more potent than etoposide against MCF‐7, SK‐N‐MC, and MDA‐MB 231 cell lines.