The state-of-the-art vessel went afloat in Berne-Motzen in northern Germany ahead of its official launch to mark Greenpeace's 40th birthday in October.
The original Rainbow Warrior, a converted fishing trawler, was sunk by French agents in New Zealand in 1985 while attempting to stop nuclear testing in the Pacific.
The second is more than 50 years old and is being retired after having been "rammed, raided and bombed" in numerous campaigns against nuclear testing, over-fishing and illegal logging, Greenpeace says.
The 58-metre-long (190-foot-long) Rainbow Warrior III is the first to be built from scratch to its own specifications.
The boat is powered by sails on its 50-metre masts, by an electric motor allowing it to reach a top speed of 10 knots (18 km/h) and a diesel engine giving it 15 knots (28 km/h).
Capable of housing a crew of up to 33 for four weeks without outside supplies, it also has a helipad in the stern and its masts will be decked out with 48 antennae and other pieces of electronic gadgetry.
"It's a symbol of our fight," said Greenpeace Germany member Christian Bussau.
"We want the Greenpeace tradition of protecting the sea to continue, and that's why, for a third time, we're calling this vessel the Rainbow Warrior."
The ship was constructed at the 161-year-old German shipyard Fassmer.