Mr Ahmadinejad was warmly greeted by Mr Chavez and will hold two days of talks with his Venezuelan counterpart and sign agreements on energy.
"We have a common thinking, common interests," Mr Ahmadinejad said after his arrival. "We must be united to accomplish these ideas, to reach the objective of peace and justice in the world."
This is the Iranian leader’s first visit to Latin America since taking office last year.
He hailed "all the free countries and liberators, all the revolutionaries who are against global hegemony."
The two leaders share a strong enmity towards the United States, which has had a war of words with Chavez and accuses Iran of seeking to build a nuclear weapon.
Mr Chavez, who has visited Iran several times, praised Mr Ahmadinejad as a "distinguished leader of a heroic people and leader of a revolution, sister of the Venezuelan revolution."
Uranium claims
The leftist Venezuelan leader denied allegations that his country was secretly extracting uranium but did not rule out nuclear cooperation with Iran in the future.
"I have already begun to fight imperialists and internal enemies who continue to say that Iran is coming looking for uranium ... to make an atomic bomb," Mr Chavez said.
"They don't tire of lying, but their attacks have run up against the power of morality and truth," he said.
Colombia's ambassador to the Organization of American States recently said Venezuela was extracting uranium in buildings disguised as bicycle factories.
However, Colombian President Alvaro Uribe later said that the envoy had made a mistake.
Mr Chavez and Mr Ahmadinejad, whose countries are members of OPEC, will inaugurate via satellite a training center for a subsidiary of
Venezuela's state-owned oil company PDVSA and a mosque west of Caracas.
They will also visit a tractor factory in south-eastern Venezuela and Mr Chavez said Iran will help Venezuela build a gunpowder and detonator factory.
Other bilateral agreements include cooperation in the construction in Venezuela of car and computer factories, and petrochemical facilities.
Mr Ahmadinejad will also travel to New York to attend Monday’s session of the UN General Assembly.