The 35-year-old said the timing was right to take the step back.
“It’s something that I’ve loved being a part of for a long period of time and I’m so grateful for the memories and experiences,” Williamson said on Sunday.
“It’s the right time for myself and the team. It gives the team clarity for the series moving forward and ahead of their next major focus which is the T20 World Cup.
“There’s so much T20 talent there and the next period will be important to get cricket into these guys and get them ready for the World Cup.
“Mitch (Santner) is a brilliant captain and leader – he’s really come into his own with this team.
“It’s now their time to push the Black Caps forward in this format and I’ll be supporting from afar.”