The five-day poll, which closed on Monday, also showed the war weighing heavily on Trump's popularity, with his approval rating dropping to 34 per cent, a return to the lowest level of the Republican's second term that was last touched in an April survey.
Only 23 per cent of Americans - including just half of Republicans - think the US is now in a stronger position with Iran compared with its position before the war, the poll found. Some 35 per cent of respondents think it is in a weaker position. The rest said they were not sure or that the US position was about the same as before.
Trump and Iranian President Masoud Pezeshkian signed a preliminary deal on June 17 that would reopen oil and gas shipping lanes that were frozen by the conflict, while easing US-led economic pressure on Iran.
The deal has led to a rapid drop in global crude oil prices, although for most Americans the price of petrol remains considerably higher than it was before the February 28 US-Israeli strikes that started the war.
Iran responded to the initial attack with strikes that shut down a fifth of the global oil trade and damaged energy facilities of US regional allies.
Only 24 per cent of Americans think the war with Iran was worth the costs, the Reuters/Ipsos poll showed. Half of poll respondents said the conflict was not worth it and the rest were unsure.
Some 63 per cent of Americans think it unlikely that the deal Trump signed will lead to lasting peace between the two countries. About half of Republicans and eight in 10 Democrats said the deal was unlikely to deliver peace. Just 18 per cent of Americans - including 10 per cent of Democrats and 34 per cent of Republicans - see lasting peace as likely.
Trump won the 2024 presidential election after promising to reduce inflation and keep America out of costly foreign wars.
Trump started his current term with a 47 per cent approval rating, but his popularity has suffered amid high rates of inflation as well as controversy over his aggressive efforts to deport people in the country illegally, which have included deadly confrontations with pro-immigration activists.
His falling popularity could weigh on his Republican allies when they defend their congressional majorities in the November 3 midterm elections.
The Reuters/Ipsos poll showed that just 17 per cent of independent registered voters said they would vote for the Republican in their district if the election were held today, compared with 34 per cent who said they would pick the Democrat.
