The 12-month extension has been confirmed as the experienced centre-back, 31, hit an appearance-related clause in his current contract.
Bartley's existing deal, signed in the summer of 2021, will now run until 2024.
The former Swansea man has fought back to be part of head coach Carlos Corberan's first-team plans and was an integral part of Albion's three-match winning run – all while securing clean sheets which had been lacking this season – prior to the World Cup break.
He scored in two of those victories, against QPR and Stoke, including the winner at Loftus Road.
That response and eye-catching performances came after a rocky campaign for Bartley, who was jeered off by his own supporters after a nightmare display in defeat against Blues and a costly late red card in the loss at Millwall.
Corberan said of the extension: "I am very pleased to have someone like him in the team with his leadership skills and performance that he is showing from when I arrived.
"For me to be working with players with a lot of commitment with the group, and with one special mentality, of course I will be pleased to keep working with them."
Bartley has made nine appearances for Albion this season under Steve Bruce, Richard Beale and Corberan and 154 appearances in Baggies colours since his switch from south Wales in the summer of 2018.
Corberan has been full of praise for the defender since the head coach took over at The Hawthorns at the end of October. The Spaniard praised the defender's mentality and said he can be one of the most influential centre-halves in the Championship.
Bartley yesterday received a nomination as Championship player of the month for November, alongside Corberan's managerial nomination.
Albion return to action after a month-long pause in the second tier at Sunderland in front of the TV cameras on Monday night. Corberan is set to face the press ahead of that clash later on Friday.
While the boss was pleased to see Bartley receive recognition in the media recently, he insisted any plaudits for his boss nomination are all down to the group of players and staff.
"In the case of Bartley, the nomination is more individual because he has had very good performances with a massive contribution to the team," added the head coach.
"This is a more individual nomination and when you are the coach it is a more collective nomination, it is not something individual.
"I am pleased to have the nomination, yes, because it means the team is winning games, this is the most important thing. When your team is winning games you receive the nomination that is not for you, it is for the dynamic of the team.
"But in this case it's in my name and it's important, not for me individually, but something as a team as we have won games."