"All United States presidents always say all options are open. But it is not on the table, it is not on the agenda. I happen to think that it is inconceivable," Straw told British Broadcasting Corporation (BBC) radio on Wednesday.
US President George Bush, whose government believes Iran intends to produce atomic weapons, has however refused the rule out military strikes.
Iran insists its nuclear programme is designed for generating electricity, but the US and others accuse it of seeking to develop atomic weapons.
Britain, France and Germany are leading European Union diplomatic efforts to persuade Iran to abandon its uranium enrichment activities.
Uranium enriched to low levels can be used as fuel in nuclear reactors to generate electricity, but further enrichment makes it suitable for a nuclear bomb.
In a sign that the established nuclear powers' patience is wearing thin, the International Atomic Energy Agency passed a resolution on Saturday putting Iran on the verge of referral to the UN Security Council unless Tehran eases suspicions about its nuclear activities.
The resolution ordered Iran to suspend all enrichment activities, including uranium conversion, to abandon construction of a heavy water nuclear reactor and to grant access to certain military locations, individuals and documents.
"The truth is, as (US Secretary of State) Condoleezza Rice has made clear, military action in respect of the Iranian dossier is not on anybody's agenda. I believe it is inconceivable"
British Foreign Secretary
Iran has rejected the resolution, protesting it was politically motivated and without legal foundation.
"The truth is, as (US Secretary of State) Condoleezza Rice has made clear, military action in respect of the Iranian dossier is not on anybody's agenda. I believe it is inconceivable," Straw told the BBC.
Straw, who is in Brighton, southern England, for the governing Labour Party's annual conference, said the IAEA resolution left the "door open for further diplomatic action with Iran" and urged the country to take that route.
He insisted the way the international community dealt with the nuclear standoff was of fundamental importance and could affect the "geopolitical landscape for years to come".
"There is no question of us going to war against Iran. Why? Because its not going to resolve the issue. No one is talking about going to war against Iran," he later told Sky Television News.
"This can only be resolved by diplomatic means and by diplomatic pressure."