The Scotland centre has been at Scotstoun since 2015 after joining from Queensland Reds and won his first Scotland cap in the 2019 Six Nations after qualifying on residency grounds.
Glasgow have not revealed the length of the contract.
Warriors head coach Danny Wilson said: “Sam’s a really exciting midfield player both sides of the ball. He’s an intelligent player who has a real understanding of the game, and he uses that to bring the best out of the players around him.”
Johnson, 27, told the Warriors website: “I’m excited to re-sign with the club. Glasgow is my home and to be able to sign for a few more years is a great boost for me.
“There are exciting times ahead for the club and I’m very fortunate for Glasgow and Scottish Rugby to give me the opportunity to continue to work hard with Danny and the rest of the coaching team to build something special at Scotstoun.”
Johnson had a spell playing rugby league in Australia before returning to union. After moving to Scotland he had stints with Premiership clubs Stirling County, Ayr and Glasgow Hawks.
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