The news is a boost for the Seagulls following the £50million sale of Ben White to Arsenal.
The 26-year-old centre-back has been a key part of Graham Potter’s side, making 60 Premier League appearances since moving from Bristol City in a £20million deal two years ago.
“I am absolutely over the moon to sign a new deal,” Webster told the club’s website. “As soon as I heard there was the potential for me to sign a new contract I was thrilled and I am pleased to get it done. Now I can focus on the new season.
“The club have shown a lot of faith in me to bring me here in the first place, to reward me with a new contract is amazing and I can’t wait for the next five years.”
Webster began his career with Portsmouth and has also played for Ipswich.