Australia batsman George Bailey will join Middlesex from the end of June to cover the mid-summer loss of compatriot Adam Voges.
Bailey, 33, who has played 71 one-day internationals and 28 T20s, will be available for part of the T20 Blast and One-Day Cup group stages.
He averages 41.63 in ODIs and 26.11 in T20Is and has captained the Australian side in both formats.
Voges is set to play in Australia's Tests against Sri Lanka this summer.
Former New Zealand captain Brendon McCullum will join Middlesex when his Indian Premier League stint comes to an end and stay until 25 June, while fellow Kiwi Mitchell McClenaghan will also join after the IPL.
"Obtaining high-quality overseas player cover for an entire season is extremely difficult and everyone at Middlesex is delighted that we have been able to acquire the services of a player of George's ability," said managing director of cricket Angus Fraser.
"George is an outstanding cricketer with an excellent record in all forms of the game and, as someone who has captained his country, he will be a valuable addition to the squad both on and off the field."
Tasmanian Bailey, who departs in mid-August, spent a spell playing limited-overs formats with Sussex last season, while he has also represented Scotland and Hampshire in English domestic cricket.