The 43-year-old former England opener retired from playing at the end of the 2019 campaign after 27 consecutive seasons at Taunton, during which time he broke a host of club records.
The prolific left-hander hung up his bat having scored the most first-class centuries (52) and List A runs (7,374) for Somerset, as well as claiming the most first-class catches (445).
He also represented England over 200 times across all three formats, scoring more than 10,000 runs at international level.
Trescothick’s new role at Somerset will see him mainly assisting second XI head coach Greg Kennis with the aim of developing players for the first team.
“I’m absolutely over the moon to have the opportunity to join the coaching set-up here at the club and I can’t wait to get my teeth in to it,” said Trescothick.
Somerset director of cricket Andy Hurry said: “The name Marcus Trescothick is synonymous with Somerset Cricket.
“His knowledge of the game and what it means to represent this club are a valuable asset and one that we are delighted that we will be able to continue to tap in to.”
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