The company installed a single 2.4MW turbine about 3km off the coast from Choshi city, Chiba prefecture.
Japan's New Energy and Industrial Technology Development Organisation (NEDO) funded two-thirds of the 3.5bn yen (43m) in project costs, with the remainder put up by Tokyo Electric Power (TEPCO).
The turbine, installed with fixed foundations, will be connected to the TEPCO grid in January 2013 using underwater cables.
Mitsubishi's 2.4MW turbine is the largest to be installed off Japan's coast. The project had been scheduled to start last year but was delayed following the March 2011 earthquake and tsunami.
Yoshinori Ueda, of Japan's Wind Energy Association, who is also assistant general manager of MHI's wind turbine business, says the deployment is "one small step compared to Europe, but one giant step for Japan".
Mitsubishi has also developed a 7MW turbine, which will be used in a demonstration floating offshore project near Fukushima, to begin construction next year.
The company is launching its European offshore push in Scotland and has been linked with a possible investment in Danish turbine giant Vestas.
Japan currently has about 2.5GW of installed wind power capacity onshore but limited space for large wind projects and a monopolistic grid system that does not allow for transmission of electricity across the country is limiting expansion of onshore wind.
Japan's Wind Power Association (JWPA) sees much more potential for offshore wind capacity.
It estimates that a possible 519GW of floating offshore wind and 94GW of fixed offshore wind compared with 169GW of onshore wind.
The country installed its first floating turbine in June near Kabashima Island in Goto, Nagasaki Prefecture.
