We synthesized dithieno[3,2-b:2′,3′-d]galloles containing four-coordinated gallium atoms. It was found that dithienogalloles had high stability to air and moisture and showed narrower energy-band gaps than dithienosiloles which are commodity materials in organic opto and/or electronic devices. In addition, relatively-higher HOMO levels were observed from dithienogalloles than those of other dithienoheteroles from electrochemical measurements. We experimentally and theoretically demonstrated the electron-donating properties and resonance effects of gallium atoms of dithienogalloles.