The franchise announced yesterday it had secured the contract of the veteran lock, and he would join up with the squad at the start of next year.
He was rumoured to be in the sights of the Highlanders three weeks ago and the move was confirmed by the franchise this afternoon.
He has 59 test caps for the All Blacks and has won virtually every honour in the game.
Highlanders coach Jamie Joseph said Thorn's professional attitude and commitment was a welcome addition to the side,
Joseph had been in Japan recently, where Thorn was playing, and said he was still playing to a high level.
Thorn, who will be 38 in February, last played in New Zealand at the World Cup last year.
His presence at the Highlanders will be a huge boost as the side looks to build to a strong 2012 campaign.
Thorn is the second All Black signing for the Highlanders in three days.
Ma'a Nonu, a 71-test midfield back, announced on Tuesday he was joining the southern franchise.
