The Netherlands international, who returned to Taunton in 2016, has signed a new one-year deal and has been signed up as a local player
Somerset have secured the services of Roelof van der Merwe until the end of the 2023 season.
The Netherlands international's deal was due to expire at the end of next term.
But he has penned a 12-month extension to prolong his second spell at Taunton, which began in 2016, and is again registered as a local player.
Across all formats, the ex-South Africa allrounder has played 144 times for Somerset.
“I’m really pleased to have extended my time at Somerset," he said. "We have a great group of guys here and I genuinely feel that we are on the verge of something exceptional.
Roelof van der Merwe has signed a contract extension at Somerset (Harry Trump/Getty Images)
“The club holds a very special place in my heart, and I will be doing everything that I can to make sure that we win games of cricket and claim the silverware that the players and our brilliant Members deserve.”
Director of cricket Andy Hurry added: “Roelof is a quality performer across the three formats of the game.
"His drive, determination and will to succeed are second to none and he is a genuine team man. He is exceptional both on the field and in the dressing room, and all his domestic and international experience make him an outstanding role model for our aspirational players to learn from.
“His passion for the game is infectious and his contributions with the bat, the ball and in the field are of the highest standard."