Khaled Meshaal also praised Mahmoud Ahmadinejad, the Iranian president, for his "courage" in having dismissed the Holocaust as a myth and calling for Israel to be moved out of the Middle East to Europe or North America.
"Just as Islamic Iran defends the rights of the Palestinians, we defend the rights of Islamic Iran," Meshaal told reporters in Tehran on Thursday. "We are part of a united front against the enemies of Islam.
"Each member of this front defends itself with its own means in its region. We carry the battle in Palestine. If Israel launches an attack against Iran, we will expand the battlefield.
"We are part of a united front, and if one member of this front is attacked it is our duty to support them."
Israel and the US accuse Iran of using an atomic energy drive as a cover for weapons development, which the Islamic republic denies.
Ahmadinejad has increased alarm in the West after calling for Israel to be "wiped off the map". On Wednesday he drew fresh condemnation after denying the massacre of Jews during World War II took place.
Meshaal said: "We congratulate Iran for its position in the Islamic world and in particular the statements of the supreme leader [Ayat Allah Ali Khamenei) and President Ahmadinejad.
"What Iranian officials say may not please some people, but these are just courageous declarations.
"The Muslim people will defend Iran because it voices what they have in their hearts, in particular the Palestinian people."
On Iran's nuclear programme, he said: "Even if Iran's programme is military, what is the problem? Why do other countries like Israel have the right to have a nuclear weapon?"