World leaders had hoped to put out a statement of consensus on the Paris Agreement which is designed to cut global carbon emissions but Mr Trump took to social media to say he would make up his mind next week.
Leaders including the German Chancellor Angela Merkel expressed frustration with the Americans.
She told reporters: “The entire discussion about climate was very difficult, if not to say very dissatisfying. We have a situation where six, or if you also include the EU seven, are against one.
“That is to say, so far there are no indications whether the United States will stay in the Paris Agreement or not.
“Therefore we didn’t beat about the bush but rather made it very clear that we six of the G7 member states plus the EU continue to support the targets.”
Many suspect President Trump-who previously called global warming a hoax, is trying to use the climate change issue as leverage in other debates but his decision left a sour taste with leaders and diplomats who are having to revisit an issue they hoped they had sorted.
The final hours of President Trump’s time in Italy were overshadowed by claims that his son-in-law and senior advisor Jared Kushner had undisclosed contact with the Russians during last year’s US election.
But there was no chance for journalists to question Trump, who broke with G7 protocol by not holding a news conference.
He had given some ground, agreeing to language in the final G7 communique about fighting protectionism and promoting free trade – language which may infuriate his supporters at home who voted for his ” America first” vision.
During the campaign he had threatened unilateral tariffs on Mexican and Chinese goods and said he would quit the North American Free Trade Agreement unless it is renegotiated to his liking.
Earlier this week he called Germany “very bad” on trade because of its surplus with the USA.
There had been pockets of protests around the G7 event in Taormina, chosen to highlight the issue of migration.
Thousands cross from Africa every week, many losing their lives at sea and the Italian hosts wanted to get agreement on a range of measure to stem the tide.
But Edmund Cairns for the charity OXFAM expressed disappointment, describing the summit as ‘a failure’.
“The summit has ended with two waffly paragraphs about refugees and migration. It’s a scandal there is no credible plan at all to protect refugees,” he said.
The final communique was just six pages long reflecting some of the areas where progress had been made.
Diplomats said on counter-terrorism, Syria and North Korea, there was broad G7 agreement.
Security questions had dominated the first day of the summit with a push by Theresa May to do more to get internet service providers and social media firms to “substantially increase” their efforts to rein in extremist content.
Donald Trump is now on his way back to America, ending a marathon nine-day tour of the Middle East and Europe – his first foreign trip since taking office.
But if he had hoped for some respite, those Russia claims are likely to mean he will be disappointed when he gets back home.