The wing-back is the third player to agree extended terms with the Baggies today – with both Matt Phillips and Kyle Bartley also signing new deals.
Furlong, who signed from QPR two seasons ago, has agreed a deal which will keep him at The Hawthorns until the summer of 2025.
His previous deal expired at the end of the 2022/23 season.
Furlong will make his 70th appearance for Albion if he starts tomorrow's Championship opener at Bournemouth.
And new boss Valerien Ismael is thrilled to have his right wing-back tied down for a further four years.
“The wing-back positions in our system and philosophy are the most intense and exposed position – out of possession and especially in possession – and Darnell has the perfect profile," Ismael said.
“When I came, I recognised straight away that he is made for the position and made for us.
“He’s perfect for us and, with his quality and if he can stay healthy this season, he will put us on another level.”