The 32-year-old has won 31 Springboks caps at wing or full-back and has also played centre.
He said: "When looking at the current squad, the average age and talent within the region, that is what got me interested.
"I am keen to find my feet, settle in with boys as quickly as possible, and add value to the squad where I can."
Kirchner will join three fellow South Africans at Rodney Parade - lock and captain Rynard Landman, scrum-half Sarel Pretorius and full-back Carl Meyer.
He will leave Pro12 leaders Leinster after four seasons.
Dragons have traditionally fared worse than their three Welsh rivals in the Pro12 and only Treviso and Zebre are below them with five games remaining for the Welsh team.
They are also the subject of talks over a change of ownership that could see the Welsh Rugby Union take over.
Kirchner believes Dragons "will definitely move in the right direction" in the future.
He added: "New decisions and ventures aren't always easy to make, however the experience of moving to Leinster, my first move abroad, has helped and knowing a few of the other South Africans in the squad makes my decision easier."
Dragons head coach Kingsley Jones said: "His experience both on and off the field will be invaluable for this young squad."

The Bulletin of the Atomic Scientists (BPA), the group behind the clock, said the standing still is "not good news".
The minute hand on the Doomsday Clock is a metaphor for how vulnerable the world is to catastrophe.
"It remains the closest it has been over the past 20 years," said Rachel Bronson, BPA's executive director.
In addition to nuclear arms and climate change, the group also cited growing cyber threats and an uptick in terrorist attacks in their decision to keep clock unchanged.
Lawrence Krauss, chairman of the BPA's Board of Sponsors said that the Iran nuclear agreement and the Paris climate accord were good news, but said it remained unclear if the Paris agreement would actually reduce greenhouse gas emissions.
He also noted increased tensions between the US and Russia as a sore point.
Last year, the scientists moved the clock up from five-minutes-to-midnight, noting the threat of climate change, the modernisation of nuclear weapons as well as large nuclear arsenals.
At the time, they said the threats were "extraordinary and undeniable threats to the continued existence of humanity".
The Bulletin of the Atomic Scientists was founded at the University of Chicago in 1945 by a group of scientists who helped develop the first atomic weapons. Their metaphorical clock was created two years later.
Today, the group includes physicists and environmental scientists from around the world, who decide whether to adjust the clock in consultation with the group's Board of Sponsors - which includes 17 Nobel laureates.
The closest the clock has come to midnight was in 1953, when it was moved to two minutes from the apocalyptic midnight, following hydrogen bomb tests by the US and Russia.
Newport Gwent Dragons have signed South Africa utility back Zane Kirchner from Pro12 rivals Leinster for 2017-18.
[NEXT_CONCEPT]
The so-called Doomsday Clock will remain set at three-minutes-to-midnight amid global perils such as climate change and nuclear proliferation.