Lynn joins head coach Felipe Contepomi and his staff ahead of the July Internationals where Los Pumas face France and Uruguay.
Fellow New Zealander Brad Mooar joined the coaching team in March but will no longer be involved for personal reasons.
Lynn takes Mooar’s place and will work alongside former Pumas Andrés Bordoy and Juan Martín Fernández Lobbe.
“We’re proud of Kenny and his appointment as a coach for the Pumas. We will certainly miss him in our coaching team; however, it’s natural, for both players and coaches, to strive for a national team,” Highlanders chief executive Roger Clark said.
“We wish Kenny, Becs and the kids the very best on their next adventure.”
An announcement about a replacement will be made later this year ahead of the 2025 season.
Former Pumas’ first five-eighth Contepomi took over from Australian Michael Cheika, after October’s World Cup third-place play-off loss, and these will be his first tests in charge.
A familiar face joins Lynn in Highlanders outside back Martin Bogado. This will be Bogado’s second year in the Argentine squad.