The 26-year-old was one of manager Graham Potters’ first signings when he took over the club in the summer of 2019.
Webster joined for a £20m fee from Bristol City and has made 60 Premier League appearances in the last two seasons, scoring four goals.
The former Portsmouth defender is expected to play an even bigger role this term following the departure of Ben White to Arsenal for £50m.
“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.”
Potter added: "Adam has been a really important figure for us on and off the pitch, so I am delighted to see him commit to the project we have here."
"Adam was one of the first players to sign after I joined the club two years ago and I am delighted with his development in that time. He has captained the team on a few occasions which is a real testament to his qualities.
"We're looking forward now to seeing him continue his fantastic form from last season into the new campaign."
Brighton will begin their 2021/2022 Premier League campaign away at Burnley on August 14.
