Nelsen has won 29 full caps for New Zealand
New Blackburn manager Paul Ince has given club captain Ryan Nelsen permission to play for New Zealand at the Olympic Games in August.
The 30-year-old will join the New Zealand Under-23 squad as one of three over-age players.
Nelsen will face China and Brazil before returning in time for Blackburn's opening Premier League match against Everton on 16 August.
"It's great news," said New Zealand's Under-23 coach Stu Jacobs.
"We've got one of the world's best defenders lining up for us in two crucial matches.
"The reality is that the club didn't have to release the player so Blackburn have really come to the party.
"They've got a new manager in Paul Ince who hasn't even met his captain yet but he said he knew Ryan was desperate to play and given the magnitude of the Olympic Games he was more than happy to help out where he could."
Nelsen has not played for New Zealand since 2004 because of a recurring hamstring injury.