Bush, who met German Chancellor Angela Merkel as part of his week-long tour of Europe before flying to Rome, is pressing allies to agree new punitive measures against Iran.
While Europeans have voiced support for new sanctions, they are also looking past Bush, whose presidency ends in January.
"Both the chancellor and my first choice, of course, is to solve this diplomatically," Bush told a joint news conference with Merkel.
But he added: "All options are on the table", a reference to the threat of military action to stop Iran's nuclear programme, which the West fears is aimed at making atomic bombs.
"The message to the Iranian government is very clear," said Bush, visiting Europe for the last time before the end of his eight years in office.
Merkel was more cautious, saying she could "not exclude" a further round of sanctions if Iran failed to cooperate and suspend enrichment work, which Tehran argues is for peaceful power generation.
With his approval ratings at home at the lows of his presidency and his domestic agenda largely blocked by an opposition-led Congress, Bush is trying to reassert his relevance on the world stage and forge a foreign policy legacy defined by more than the unpopular war in Iraq.
He has focused increasingly on Iran and says he wants to leave his successor a framework of international diplomacy for tackling the Iranian nuclear threat.
Despite three rounds of sanctions by the U.N. Security Council, Iran has refused to stop enrichment.
This weekend, EU foreign policy chief Javier Solana will be in Iran to present a revised package of political and economic incentives for Iran to give up enrichment, similar to an offer made in 2006 that was rejected.
Italy, where Bush arrived later on Wednesday for the next leg of his tour, promises a tougher line on Iran now that Silvio Berlusconi, a firm Bush ally, has returned as prime minister.
