The semi-official Mehr news agency said the one-day trip to Kabul would be Ahmadinejad's first visit to Afghanistan since both he and Karzai were re-elected last year.
Karzai had invited Ahmadinejad and the visit was aimed at expanding bilateral ties, Mehr added. They would also discuss "solutions for settling the problems" in Afghanistan.
Western powers want regional players to cooperate in bringing stability to a country where U.S. and other foreign troops back Karzai's government in the face of an insurgency by the Islamist Taliban.
Iran says the presence of foreign troops in Afghanistan is a key reason for the problems in its eastern neighbor.
Western forces have been in Afghanistan since 2001, when the United States led an invasion to drive the Taliban from power over their alliance with al Qaeda.
Western security analysts have long talked of the need for a regional settlement on Afghanistan to prevent a resurgence of old rivalries which could stoke a renewed civil war when U.S.-led troops begin to leave.
But Tehran, locked in a showdown with the United States over its nuclear program, has little reason to cooperate with Washington in helping it stabilize Afghanistan.
(Reporting by Reza Derakhshi and Hashem Kalantari; writing by Fredrik Dahl)
