"Since we arrived last year, Jaquiski has impressed us with his play, his work ethic and the kind of teammate he is. We believe he's a natural fit in our defensive system, and his abilities will allow him to flourish. It is always exciting when we are able to extend our own young players like Jaquiski who are dedicated to their craft, this team and our community."
Ian Rapoport of the NFL Network reported the new deal is worth a maximum of $15 million.
Tartt has emerged as one of the league's most undervalued safeties since the 49ers selected him in the second round of the 2015 draft.
The 26-year-old Alabama native racked up 55 combined tackles, three passes defended and one interception across nine appearances last season before his season ended in November because of a broken arm.
Pro Football Focus graded him as the NFL's 15th-best safety for 2017.
David Lombardi of The Athletic noted the extension comes one day after the Niners passed on the position's top prospects in the 2018 NFL draft:
David Lombardi @LombardiHimself
The 49ers had a chance to draft a high-profile safety prospect yesterday. Both Derwin James and Minkah Fitzpatrick were available at No. 9. The 49ers took McGlinchey instead. Jaquiski Tartt and Adrian Colbert are their two starters at safety right now. The team really likes both.
Tartt has the potential to push for his first Pro Bowl selection if he stays healthy next season and should remain a stalwart within the San Francisco defense for the foreseeable future.