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."
Newport Gwent Dragons have signed South Africa utility back Zane Kirchner from Pro12 rivals Leinster for 2017-18.