He said Japan was already one of New Zealand's largest trading partners and one of the region's economic heavyweights.
"Japan's participation in TPP would add substantial economic weight to the group. More significantly, it would reinforce the strategic vision we have for TPP, including the agreement's potential to serve as a platform for wider trade and economic integration across the Asia-Pacific."
After TPP talks in Honolulu in 2011, US President Barack Obama said he expected an agreement would be completed by the end of this year.
Mr Groser said the eleven other countries - Austalia, Brunei Darussalam, Canade, Chile, Malaysia, Mexico, New Zealand, Peru, Singapore, the United States of America and Vietnam - were assured by Japan's commitment to achieving a comprehensive and ambitious agreement as soon as possible.
Japan will formally join after those countries have made the required legal changes.