Williams has already won 24 caps for Wales and will start for his country against Ireland on Saturday in the RBS 6 Nations encounter.
WRU Group Chief Executive, Roger Lewis, said: “We are delighted Scott Williams has decided to commit his future to the Region and Wales.
“The priority for us all is to keep as many key players in Wales as possible as we all understand the pressure many of them have to move abroad.
“Scott is a fantastic player and a great ambassador for our game off the field of play. He is a proud Scarlet who is a real favourite with the supporters.”
The WRU and the Regions are currently negotiating a new Rugby Services Agreement which will include terms for player welfare and release for international duty.
The WRU has declared the governing body is prepared to negotiate National Contracts with key players the Regions have been unable to negotiate renewed contracts with so they can remain in Wales with their region.
