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The centre joined the Cherry and Whites from Worcester Warriors in 2022 and has since become a key player for the club.
Atkinson was named Supporters' Player of the Year in both 2023/24 and 2025/26.
His performances earned him a senior England debut against Argentina in 2025.
He has since become a regular in Steve Borthwick's England squad.
Gloucester head coach George Skivington said: "This is a really significant re-signing for us.
"Seb had plenty of options on the table, so his commitment to the club is a real indication of where Gloucester Rugby is right now."
Atkinson said: "Playing at Kingsholm is a privilege, and I've been overwhelmed by how the Gloucester supporters have welcomed me and made me feel one of their own."
He added that he remained committed to helping Gloucester achieve success.