He joins his former teammate and much-loved former All Blacks halfback Piri Weepu, who has posted a number of posts against the TPP.
Weepu this week warned the signing of the TPP was coming next month.
"Getting closer and closer to the rubbish the National party are getting us in to. #TPPANoWay #SayNoToTPPA," he tweeted.
Ministers from the 12 countries that have negotiated the comprehensive and controversial deal will formally sign the agreement.
The signing will be a high-profile celebration, but the deal will still need to be ratified in national parliaments. In the United States, the Obama administration had aimed to have the deal ratified by July.
The secretive TPP trade deal was finalised in October, with the 12 countries agreeing the deal after five years of intense negotiation.
The pact will cut trade barriers and set common standards for 40 per cent of the world's economy, and deliver New Zealand its long sought-after free trade deal with the US, the largest economy in the world and Japan, the third-largest economy, as well as Canada, Mexico and Peru.
Other TPP players are Australia, Singapore, Brunei, Chile, Vietnam and Malaysia, all of which New Zealand has deals with.
TPP talks began in 2010 but strong public opposition to the deal here has centred on concerns about its impact, such as making pharmaceuticals more expensive and a loss of sovereignty. Thousands of people demonstrated in August, many angry that details of the deal had been kept secret.
Shortly after being part of the All Blacks' World Cup-winning squad, Messam moved to Japan and is currently playing with the Tokyo-based Toshiba club.
Other rugby players to post political messages on social media include Israel Dagg and the late Jonah Lomu, who both broke the law when they expressed support for National on election day in 2014.
Publishing anything on election day which could potentially influence another voter is prohibited until voting booths have closed under the Electoral Act 1993.
