The developers of a planned tidal energy scheme in the Inner Sound of the Pentland Firth have signed a multi-million pound deal to sell electricity generated by the project.
MeyGen said it had signed a 10-year Power Purchase Agreement (PPA) with energy firm SmartestEnergy..
Under the first phase of the project, four 1.5MW turbines will be installed on the seabed.
MeyGen said eventually the scheme could have up to 269 turbines.
It plans to start installing devices over the next two years.
MeyGen, owned by Atlantis Resources, said the PPA agreement could be worth more than £50m over its lifetime.
The agreement with SmartestEnergy will provide revenues from power generated and exported to the grid under the first phase of the project through to 2025.
Atlantis announced recently it had secured a £50m funding package for the project from a range of backers, including the Scottish government, Highlands and Islands Enterprise, the UK government and the Crown Estate.
MeyGen chief executive Dan Pearson said: "This is an important step forward for MeyGen and its backers.
"The agreement with SmartestEnergy will secure the long-term revenues which will demonstrate the commercial viability of the project and also highlight the wider potential of tidal stream generation to become a significant industry."
WWF Scotland director Lang Banks said it was a major boost for the marine renewable industry in Scotland.
He said: "The sooner we can demonstrate devices on a commercial basis the greater the chance Scotland will have at becoming a world leader in developing the technologies to turn tidal power into clean, green electricity.
"Alongside energy saving measures, marine renewables will have a critical role to play in helping Scotland reduce climate emissions as we phase out polluting fossil fuels and nuclear power."