"We are striding on a common path towards a brighter future and will remain by each other's side and supportive of one another under any circumstances," he said.
While both countries are energy producers, Bolivia is one of several south American countries that have forged closer relations with Iran.
Iran's president had already promised $1.1bn in aid for impoverished Bolivia, which is trying to build up its gas industry.
Morales, who in 2006 became the first indigenous leader of Bolivia, said: "I support and praise Mr Ahmadinejad's stance against imperialism and defending the rights of the Iranian people.
"I also hail Iranian progress in industry and agriculture," he said.
Bolivia and Iran had established relations in September 2007 when Ahmadinejad made an official trip to Bolivia to sign trade and energy accords.
Their growing ties have raised concerns in the US.
The US has led international efforts to isolate Iran, which it accuses of seeking atomic weapons under the cover of a nuclear energy programme.
Tehran denies it has any intention of seeking to build a nuclear bomb, saying its nuclear programme is intended to produce electricity.