The 32-year-old Sangakkara, one of the world's leading batsmen, will arrive at Old Trafford once he has completed his international commitments at the World Twenty20 in the Caribbean and at the proposed Asia Cup.
Sangakkara has been a mainstay of the Sri Lanka side for a decade and the left-hander has an impressive Test batting average of 55.10.
He is also a top-class wicketkeeper although Lancashire said he had been signed as a batsman.
Sangakkara is set to become the latest Sri Lankan to represent Lancashire, following international team-mates Muttiah Muralitharan and Sanath Jayasuriya.
Speaking from India, where Sri Lanka have just completed their latest tour, Sangakkara, in a Lancashire statement, said: "I am thrilled to be joining Lancashire and really looking forward to representing a club and county that has such a long and distinguished cricketing history.
"Murali always talks fondly of his times at Lancashire so I had no hesitation in accepting the club's kind of