The vastly experienced Sri Lankan will be available for the first seven matches of Sussex’s campaign, beginning with the match against Kent Spitfires on Friday, May 15, at Canterbury.
The 37-year-old has played 170 matches in the shortest format of the game, scoring over 4000 runs and has figured 55 times for his country in International T20 matches, scoring nearly 1,500 runs, with his highest score of 100 coming against Zimbabwe in 2010.
He retired from international T20 matches last year and signed off in style by helping Sri Lanka win the World Twenty20.
One of their most decorated players, Jayawardene has scored over 10,000 runs in both Test matches and One-Day Internationals for Sri Lanka.
Jayawardene 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.”