Ashwin comes in for New Zealand’s Kyle Jamieson, whose short Surrey stint has been ended a game early by a glute injury picked up in the draw against Hampshire.
Surrey are also without Ollie Pope, who is out until August with a grade three thigh strain, Will Jacks, who is with the England ODI team for their series against Pakistan.
Ashwin, 34, is a fine signing by Alec Stewart and his team. He has 413 Test wickets, as well as five centuries.
Ashwin has previously played county cricket for Worcestershire and Nottinghamshire, and will use the fixture as a warm-up for India’s five-Test series against England in August.
He will also be a useful mentor for Amar Virdi, the young Surrey off-spinner who has spent time in the England bubble over the last year, and will come up against Jack Leach, who is back in Somerset’s squad.
Somerset top the Championship’s Group 2 going into the final round of fixtures. Surrey need a comprehensive victory, and to hope that Gloucestershire’s meeting with Hampshire goes their way, to sneak into Division One.