McCullum will lead New Zealand in England this northern summer then will stay on for his stint in the NatWest Blast beginning on June 26, Cricinfo reports.
The hard-hitting Kiwi will play seven games for the reigning champions.
"Brendon's dynamic performances in the World Cup have demonstrated once again exactly why he is one of the most feared batsmen in the game," Dougie Brown, Warwickshire director of cricket, said.
"Securing his availability for our last seven games has to make him one of the biggest overseas player signings in the competition's history.''
McCullum, who played T20 for Sussex in 2010 and Glamorgan in 2006, joins Aaron Finch, Glenn Maxwell, Tillakaratne Dilshan, Martin Guptill, Grant Elliott, Mahela Jayawardene, Kumar Sangakkara and Chris Gayle as World Cup stars who will play in this season's NatWest Blast.
