Williamson, who led New Zealand to victory in the inaugural World Test Championship final against India at the Ejaz Bowl in 2021, will continue as the white-ball captain.
“It has been an incredibly special honor to captain the Black Caps in Test cricket,” he said. “For me, Test cricket is the pinnacle of the game and I have enjoyed the challenges of leading a team in the format.
“Captainship comes with workload on and off the field and at this stage of my career I feel the time is right for this decision.
“After discussions with NZC, we felt it was best to continue captaining the white-ball formats with two World Cups in the next two years.
“I’m excited to have Tim as captain and to support Tom. [Latham] as vice-captain. Having played with both of them for most of my career, I’m sure they’ll do great.
“Playing for the Black Caps and contributing in all three formats is my top priority and I look forward to the cricket ahead.”
Southee, who has previously captained 22 T20Is, said: “It’s been an unreal few days and it’s a great honor to be appointed as Test captain.
“I love Test cricket, it’s the ultimate challenge and I’m excited about the opportunity to lead the team in this format. Kane is a fantastic Test captain and I look forward to working with him. [coach] Gary [Stead] How can we build on that?”
More to follow.