The new deal, which begins in June 2018, will also see BT continue as main sponsor of the Scotland national team and Scotland sevens side.
But there are changes to the domestic part of the contract and Scottish Rugby will now seek a new sponsor for its league and cup competitions.
The SRU’s original partnership with BT was a four-year deal, beginning in 2014, and was reportedly worth £20 million.
The most eye-catching part of the contract was the sale of the naming rights for the national stadium which saw it rebranded as BT Murrayfield.
No figures for the latest extension have been released but senior figures within the SRU are delighted with the deal, particularly with it coming during tough economic conditions.
One source said, “it remains a record-breaking deal for Scottish rugby which will allow us to invest at a record level”.
A statement from Scottish Rugby said: “Since 2014, BT’s investment has helped redefine the rugby landscape in Scotland through investment in the BT Sport Scottish Rugby Academy, BT league and cup competitions, as well as supporting the Club Sustainability Fund to provide investment in capital projects at grassroot clubs.
“BT’s distinctive logo will continue to be displayed on the front of Scotland rugby shirts across the world, in the Six Nations Championship, as well as the summer and autumn test matches.
“The partnership continues to see BT promoted at Scotstoun Stadium in Glasgow, Edinburgh Rugby and BT Murrayfield Stadium.
“Scottish Rugby has enjoyed great success on the pitch during the first term of BT sponsorship, culminating in 2017 with Scotland reaching fifth in the world rankings, Scotland U20s securing their highest ever finish in the Junior World Championships, Scotland Women recording their best Six Nations since 2010 and Scotland sevens winning back-to-back London HSBC World Rugby Sevens Series titles.”
Scottish Rugby’s chief operating officer, Dominic McKay, said: “This new deal will enable Scottish Rugby to continue to move forward on and off the pitch and clearly demonstrates we are open for business, with a fantastic product we are very proud of.”