Left-arm seamer Tanvir, 30, brings plenty of experience to the Somerset ranks, with 170 domestic and 42 international Twenty20 appearances.
His return of 6-14 for Rajasthan Royals in the 2008 Indian Premier League was the record bowling performance in T20 cricket until Somerset’s Arul Suppiah bettered it with 6-5 three years later.
Tanvir has previously played in England with Hampshire and is well known to the west country club’s director of cricket Matthew Maynard, after the pair worked together in the Caribbean Premier League.
Maynard said: “Sohail is a quality top-order bowler and a dangerous lower-order batsman. There is no doubt he will be an extremely useful addition to the squad.
“He is an experienced international cricketer who will provide us with something a little bit different. I worked with him at St Lucia Zouks and he was our star performer so I am looking forward to teaming up with him again.”
In T20 cricket Tanvir took 36 wickets at 28.52 for Pakistan, as well as playing 62 one-day internationals and two Tests.