James Mattis’ words come as Iraq gears up to decide who country’s leader will be for the next four years.
Among Iraqi Prime Minister Haider al-Abadi’s challengers are former Prime Minister Nuri al-Maliki and Hadi al-Amiri, a former transportation minister.
Both Maliki and Amiri are among Iran’s closest allies in Iraq.
Mr Mattis said: “We have worrisome evidence that Iran is trying to influence – using money – the Iraqi elections.
“That money is being used to sway candidates, to sway votes.
“It’s not an insignificant amount of money, we believe.
“And we think it’s highly unhelpful.
“Iran is following Russia’s example of mucking around in Iraq’s elections.”
The US Defence Secretary did not say whether he thought Iran had been successful or whether he believed Iran sought to undermine Abadi.
Iran has in the past denied interfering in Iraq.
Mr Mattis said his trip to Afghanistan and the Middle East had reinforced his concerns about Iran’s activities in the region.
He believes Iran is escalating violence in Syria and is aiding insurgents in western Afghanistan.
The US has accused Iran of escalating the civil war in Yemen.
The Trump administration believes Iran is threatening to turn Yemen’s civil war into a broader regional conflict by supplying advanced weaponry, including missiles, to Houthi rebels who have fired at targets in Saudi Arabia.
Mr Mattis also warned that Iran was using Yemen as a testing ground for its weapons.
He added: “It’s where you find their radars, their ballistic missiles, their anti-ship cruise missiles.
“We found their mines, their explosive boats all being tested.”
His words have come after Israeli Prime Minister Benjamin Netanyahu earlier this week told Donald Trump he still has time to correct the terms of Iran’s controversial nuclear deal with the West.
Speaking to Fox News host Mark Levin, Mr Netanyahu claimed Iran had been “let out of its cage” with carte blanche to “conquer countries” – and urged tough restrictions to be put in place until it “changes its behaviour”.