The 38-year-old, who scored more than 12,000 runs in one-day internationals, will feature in 12 of the county’s 14 T20 Blast group games this summer.
It means Somerset boast the World Twenty20’s top two all-time run scorers in Jayawardene and Chris Gayle.
“When a player of his experience and class becomes available, you have to snap them up,” said the club’s director of cricket Matt Maynard.
“Mahela is a batsman of undoubted class who scores his runs all around the wicket.
“He is a true professional and will have a major influence both on and off the field for us.”
Jayawardene, who has a Test high score of 374, has played in 55 international Twenty20 matches, scoring 1,493 runs.
He said: “The club has a tradition of successful overseas players and I will be doing my best to join that illustrious list.”
