The letter has been more than 1700 Facebook 'likes', and has been shared more than 600 times, and has attracted close to 250 comments, since it was posted less than a day ago.
Opening his letter with "Hey hey, mah bruddah!!", Mr Harawira goes on to ask Mr Obama if he "had a good catch up with the whanau" when he visited his "dad's turangawaewae" in Kenya last week.
But he quickly moves onto more serious matters, urging Mr Obama to "hold off" on the TPPA.
"You might not know who I am and that's OK because I'm not that important in the scheme of things, but my people are," he wrote.
"We're Maori, the indigenous people of a little country down here in the South Pacific called Aotearoa - you probably know it as New Zealand - and we want you to know that we don't support the TPPA; in fact we hate the bloody thing."
The letter goes on to explain how Maori have been trying to win back their rights under the Treaty of Waitangi since it was signed in 1840, but explains "it hasn't been easy".
"In fact, today our people suffer the same levels of deprivation in housing, justice, employment, education and health as Native Americans.
"Yeah bro' ? it's that bad. There's a long, long way to go before we get up to where we should be, and key to all of that is our Treaty, and our treaty rights.
"We want to be able to look after our lands, our forests, our rivers and our seas just like the Treaty said we could, not just for Maori but for everyone in this country, and we're really scared that the TPPA is going to make our fight an impossible one."
Mr Harawira expressed his concern that Maori were not able to look over the TPPA before it was signed off, and said he was worried it would compromise "our sovereignty as Maori and as New Zealanders".
"We got our own problems down here brother, and they're tough enough to deal with without having this dumped on our heads, but if we have to fight this TPPA thing we will, and we'll fight it till we beat it.
"So how about givin' us a break? Scrap the TPPA and let's start again. In fact, come on down to Aotearoa and lets go fishing, have a feed and a few beers, and between the two of us I betcha we can come up with something that works."