The 37-year-old batsman, who will be available for Sussex’s opening seven matches of the competition, has scored more than 4,000 runs in 170 T20 matches during his career, including almost 1,500 runs in 55 T20 internationals.
He was part of the Sri Lanka side that won the ICC World Twenty20 title last year – having also helped them reach the final in 2012 and 2009 – before announcing his retirement from the shortest form of the game at international level.
Jayawardene has also scored more than 11,000 runs in both Test and one-day international cricket during a glittering career, leaving him in the top seven of all-time runscorers in both formats. He retired from Test cricket last year and will stop playing ODIs after the ongoing World Cup in Australia and New Zealand.
The former Sri Lanka captain, who has never played county cricket, said: “I am really exciting about basing myself down at Hove and playing for Sussex during the first half of the summer.
“Sussex is a great club with a proud history and I want to help them win the NatWest T20 Blast this year.”
Sussex’s professional cricket manager Mark Robinson said: “It is fantastic news that a player of the calibre of Mahela has agreed to join us.
“His record and his reputation is second to none, and he will also bring into our dressing room his leadership and experience. I’m sure he will be a huge asset to Luke Wright as the new captain, and the whole squad and coaching staff.”
Sussex begin their Natwest T20 Blast campaign with the short trip to Kent on Friday, May 15.