The home of Scottish rugby is to be renamed BT Murrayfield Stadium as part of a sponsorship deal believed to be worth around £20m.
Announcing the deal, Scottish Rugby chief executive Mark Dodson described it as a "momentous day", with money to be reinvested in four new academies.
As part of the four-year partnership, the communications giant will also sponsor Scotland 7s.
BT will also back Scotland's club league and cup competitions.
Previous club competition backer RBS will continue as national team sponsor.
"We are very proud to bring BT back into our family of sponsors on a size and scale that is unprecedented both for Scottish Rugby and, we believe, Scottish sport," said Dodson.
He recognised that the headlines would be about the stadium's naming rights but stressed that the importance would be felt at grassroots level.
"This new funding underlines our desire to deliver more opportunities for young people to take up our game and broaden the reach of rugby across Scotland," said the chief executive.
"We will invest the monies that we receive from BT alongside the revenues that come from our other partners to continue our plan to grow the game of rugby throughout Scotland and deliver against our strategic plan and our recent policy paper initiatives - in particular, the establishment of a robust academy structure and to enhance the standard of club rugby."
BT Sport has been sponsor of Scotland's two professional rugby teams, Edinburgh and Glasgow Warriors, since the start of this season.
Scottish Rugby director of commercial operations, communications and public affairs Dominic McKay added: "In branding our home, the BT Murrayfield Stadium it reiterates our joint view that the name of the home of Scottish rugby that has been synonymous with the game in our country since 1925 had to be retained.
"To secure a deal of this nature for the first time in our history means a huge amount for Scottish Rugby as it will deliver very positively for us for years to come to enable us to continue to invest in our game throughout Scotland.