Iran, along with China, is one of the biggest supporters of Sudan’s President Omar Hassan al-Bashir who faces indictment from the International Criminal Court (ICC) over alleged war crimes in the long-running Darfur conflict.
Shunned by the West, Sudan, which is facing an economic crisis after losing much of its oil wealth to newly-independent South Sudan, is keen to expand trade ties with Iran. Most Western firms avoid Sudan due to a tough U.S. trade embargo.
Sudan and Iran have also strong military ties after signing a cooperation agreement in 2008.
During his two-day visit Ahmadinejad will meet Bashir on Monday and give a speech in the capital Khartoum, officials in the Sudanese foreign ministry said.
Bashir has supported the Iranian nuclear programme which Washington says is to build nuclear weapons, a charge Tehran denies.