The remarks by Alaeddin Broujerdi, the head of Iran's parliamentary committee on national security and foreign policy, came after US Vice President Joe Biden signaled that Washington would not try to prevent any such Israeli assault.
"Both the US and Israel are aware of the consequence of an erroneous decision," Broujerdi told reporters at the Iranian Embassy in Tokyo.
"I believe our response will be real and decisive," Broujerdi said. He declined to elaborate.
Israel fears Iran is developing nuclear weapons to target the Jewish state. Iran denies it is pursuing an atomic arsenal, saying it only wants to produce nuclear power.
Israel's government has said it would prefer to see Iran's nuclear program stopped through diplomacy, but that it cannot rule out a military strike.
In an interview on ABC's "This Week" on Sunday, Biden was asked whether the US would stand in the way if Israel – viewing the prospect of an Iranian nuclear bomb as a threat to the existence of the Jewish state – decided to launch a military attack.
"Look, we cannot dictate to another sovereign nation what they can and cannot do," he said.
Israel's Deputy Foreign Minister, Danny Ayalon, said neither the United States nor Israel could allow Iran to gain a nuclear weapon.
"The US, like Israel ... has determined unequivocally that Iran must not have nuclear military capability," Ayalon told Israel's Army Radio.
"A military operation in Iran is something difficult and complex and perhaps would have severe consequences and there could be serious damage, but this is much less dangerous and complicated than to allow a nuclear Iran," he said.
'Totally peaceful situation in Iran'
Broujerdi also defended a recent crackdown on protesters following Iran's presidential election.
Opposition leader Mir Hossein Mousavi has said the June 12 election, in which incumbent President Mahmoud Ahmadinejad was declared winner, was illegitimate and marred with fraud. Riots and protests have followed, although Iran's restrictions on media coverage have made it difficult to confirm some reports.
Broujerdi said Iranian police had merely acted to restore order, and accused Mousavi of instigating the protests.
"There is no confusion. It is (now) a totally peaceful situation in Iran," he said. Broujerdi is visiting Japan as chairman of the Iran-Japan Parliamentary Friendship League.
The Guardian Council, Iran's top electoral oversight body, pronounced the election results valid last week. Ahmadinejad is set to be sworn in later this month for a second four-year term.
Reuters contributed to this report