The broadcast said President Mahmoud Ahmadinejad and Cabinet ministers decided Sunday night to build the reactor in Khuzistan province, southwestern Iran.
Previously Iran had said it would build a second power plant at Bushehr, where its first nuclear reactor is due to begin generating electricity in 2006.
Khuzistan province was the site of a French-built power plant that began in the mid-1970s and was stopped after 1979 Islamic revolution.
The Iranian parliament is seeking the construction of 20 nuclear power plants. Russia, which built the Bushehr reactor, has offered to build more nuclear plants in Iran.
Iran is underto curb its nuclear program, which the United States claims is part of an effort to produce weapons. Iran says its program is limited to generating electricity.
The International Atomic Energy Agency has warned Iran that its nuclear program could be referred to the U.N. Security Council, which has the power to impose sanctions on the country.
But on Sunday, Israel's military chief of staff said he doubts diplomatic pressure will put a halt to Iran's nuclear ambitions, though Prime Minister Ariel Sharon said Israel was not spearheading efforts to block Iran from becoming a nuclear power.
"It's clear we can't have a situation where Iran will become a nuclear power," Sharon said, repeating Israel's position. "Israel is not leading the process, but it is definitely a partner to countries concerned by this dangerous development."
Military chief Lt.-Gen. Dan Halutz said he doubted attempts to negotiate Iran away from its nuclear ambitions would succeed.
"I believe that the political means that are used by the Europeans and the U.S. to convince the Iranians to stop the project will not succeed," Halutz told foreign journalists in Tel Aviv.
On Saturday, Iranthat would block international inspections of its nuclear sites if it were referred to the Security Council. The step strengthens the government's hand in resisting international pressure to permanently abandon uranium enrichment, a process that can produce fuel for either nuclear reactors or atomic bombs.
While Iran has frozen its enrichment program, it restarted uranium conversion, a step toward enrichment, in August.
The United States and European Union want Iran to permanently halt uranium enrichment. But Tehran says the Nuclear Non-Proliferation Treaty allows it to pursue a nuclear program for peaceful purposes. It has said it will never give up the right to enrich uranium to produce nuclear fuel.