The 30-year-old is the second Baggies player to commit his future to the club today – with Matt Phillips earlier agreeing terms on a deal until 2024.
Bartley made 30 Premier League appearances last season and looks set to play a key role again in the coming term under new boss Valerien Ismael.
“I think Kyle will be a future manager because he’s straight with us," Ismael said.
"After our first few days together we discussed a lot about tactics and the way we want to play.
“We spoke with him like a staff member, because he understood and tried to reproduce it on the pitch with a voice to give comments to the guys.
“He is a leader. His attitude is unbelievable and he’s very professional. He takes care of the other players and the atmosphere in the changing room.
“For the club, you can’t put a number on the worth of such a player. I’m really looking forward to working with him.”