Tidal energy specialist Nova Innovation said this was the first offshore tidal array in the world to deliver electricity to the grid.
Two 100kW turbines have been installed so far in the Shetland Tidal Array at Bluemull Sound.
The blades for the turbines were made by Shetland Composites.
Edinburgh-based Nova Innovation said tidal energy was a "long-term source" of predictable renewable power, with the turbines generating to full power across all tidal conditions.
Simon Forrest, managing director of Nova Innovation, said: "We are absolutely delighted to be the first company in the world to deploy a fully operational tidal array."
Mr Forrest said the deployment of the second turbine showcased the technology.
The UK Carbon Trust estimates a Â£126bn global tidal energy market could be developed by 2050.
WWF Scotland said the turbines were another "major milestone" on Scotland's journey to becoming a "fully renewable nation".
The charity's director in Scotland, Lang Banks, added: "With some of the most powerful tides in Europe, Scotland is well placed to lead in developing this promising technology, which will help to cut climate emissions and create green jobs right across the country."
Jenny Hogan, director of policy at Scottish Renewables, said: "Scotland is already at the forefront of capturing power from the tides and waves, and Nova's latest news demonstrates that lead is well-deserved.
"The country is already home to some of the most advanced marine energy technologies anywhere, as well as the European Marine Energy Centre: arguably the most advanced marine energy proving site in the world.
"With companies like Nova and others all working on developing this cutting-edge technology, the sector holds huge promise for the future."
Turbines installed off the coast of Shetland could herald a "new era" in tidal energy, according to the company running the project.