The turbine, which will be targeted at both the UK's Round 3 and France's 3GW offshore projects, features the world's largest wind-turbine rotor at 150-metres.
Alstom has installed the Haliade prototype at Carnet in the Loire-Atlantique region. It will undergo a series of tests over the next year before a second turbine is installed in Belgian waters.
Serial production is planned for 2014.
The turbine is currently installed near Saint-Nazaire in the Loire estuary. Alstom said this provides similar to wind conditions to what the turbine is likely to face offshore.
The prototype launch comes as Alstom attempts to build momentum behind its tender for some of France's 3GW of offshore projects. It is part of a consortium with EDF and Dong Energy bidding for the Saint-Nazaire, Saint-Brieuc, Courseulles-sur-Mer and Fecamp sites in the Atlantic and English Channel.
The Haliade turbine concept consists of a front-mounted, air and water-cooled, direct-drive permanent-magnet generator with a 7.5-metre diameter.
Similar to Alstom's geared 1.67MW and 3MW models, the 6MW turbine incorporates a drive solution that fully separates rotor bending moments and rotor torque.
So far, manufacturers to have built next generation offshore prototypes include Siemens and Repower. Further machines are still expected from Nordex and Vestas.
The company has chosen Saint-Nazaire and Cherbourg as the manufacturing sites for its upcoming 6MW offshore turbine.