The 28-year-old right-armer will arrive for the side’s final seven games following Sri Lanka’s one-day and T20 series against Pakistan.
Director of cricket Angus Fraser said: “In 2011 we had a disappointing T20 and we’re looking to react positively and improve on last season’s performances.
“The addition of Lasith can only help us achieve this goal.”
Malinga, who is currently playing for Mumbai Indians in the Indian Premier League, will start his T20 campaign with Middlesex against Essex on 21 June.
The Sri Lanka bowler, who has taken 151 wickets in 108 matches at an average of 16.4, is currently the tournament’s top bowler with a 20-wicket haul.
Fraser added: “We’re all naturally thrilled about the prospect of a player of Lasith’s talent coming to the club.
“In T20 cricket every county is looking for a bowler that provides their attack with variety, the ability to bowl during powerplays and at the death, basically something different from the norm and in Lasith we will have the best in the world. ” (BBC Sport)