The prototype dithiolene (ethane-1,2-dithione; dithio-glyoxal) constitutes the core component of metal-dithiolene complexes whose studies have burgeoned during the past half of a century in interdisciplinary research fields ranging from material sciences to enzymology. The recent vigorous research activity owes much to the works in the incubating period of the 1960s. However, due to the extreme reactivity of dithiolenes, basic studies on their electronic structure are conspicuously meager. In this paper, we report the electronic absorption spectrum of the radical anion in the title together with quantum chemical analyses on its electronic structure and the molecular geometry. As a result, the experimental result is consistently accounted for. We expect the results to be helpful for further understanding the electronic structure and reactivity of metal-dithiolene complexes.