Southee, who is currently playing for Mumbai Indians at the Indian Premier League (IPL), will join Middlesex in early July and is scheduled to be available for 13 of their 14 group matches, plus the knockout stages if required.
The 28-year-old will play alongside Brendon McCullum and Daniel Vettori, who have already been appointed Middlesex captain and coach respectively for the competition.
Excited for joining Middlesex, Southee said it wasn't a tough decision for him to make.
"When the opportunity came up to play for Middlesex it wasn't a tough decision to make and I'm looking forward to calling Lord's home while I'm there," said Southee.
Meanwhile, Angus Fraser, Middlesex's managing director of cricket, heaped praise on the New Zealander, saying that Southee is one of the leading fast bowlers in the world and has plenty of experience in T20 cricket
"Tim is one of the leading fast bowlers in the world. He has plenty of experience in T20 cricket and looks to bowl in the periods where we feel we could be better," Fraser said.
"We look forward to welcoming him to Lord's," he added.
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