The 29-year-old - who captains Gareth Jenkins' outfit - has made 109 senior appearances for the Scarlets, since linking up with them in 1999 - 83 of which were for the original RFC club.
Easterby earned a milestone 40th Test cap in Ireland's 19-13 RBS Six Nations defeat of England last Sunday - incredibly his seventeenth consecutive outing for coach Eddie O'Sullivan.
Speaking at the announcement of Easterby's renewal, Scarlets Chief Executive Stuart Gallacher said: "We are pleased to announce that Simon has signed with the region on a further three-year contract. Simon is the consummate professional and has been indispensable as captain in a season that has not gone to plan."
Easterby - younger brother of Leinster and Ireland scrum-half Guy, added: "We've had our work cut out this season and things haven't gone our way - but we're a strong squad and I believe we'll come back fighting.
I'm looking forward to the challenges of next season and proud to remain a member of the squad."