Recently, 3‐hydroxy‐1,5‐dihydro‐2H‐pyrrol‐2‐ones have become an object of close scrutiny in medicinal chemistry due to their diverse biological activity. In the present paper, we describe a substrate‐switchable reaction of hetareno[e]pyrrole‐2,3‐diones with aromatic and aliphatic thiols that afforded two series of novel 5‐thio‐substituted 3‐hydroxy‐1,5‐dihydro‐2H‐pyrrol‐2‐ones. The reaction of pyrroloquinaxolinetriones with thiols led to the formation of adducts. The initially formed adducts of pyrrolobenzoxazinetriones with thiols were subjected to hydrolysis with the oxazine ring opening and decarboxylation. Based on the latter reaction, a synthetic approach to pharmaceutically interesting monocyclic non‐fused 3‐hydroxy‐1,5‐dihydro‐2H‐pyrrol‐2‐ones bearing conformationally free thio‐substituents at C‐5 was proposed for the first time. The synthesized compounds were screened in vitro for their antimicrobial activity. Some of them were found to exhibit moderate activity against S. aureus and C. albicans. Thus, the results of current study confirm that the search for new biologically active compounds among 5‐thio‐substituted 3‐hydroxy‐1,5‐dihydro‐2H‐pyrrol‐2‐ones is promising.