After starting his first presidential trip abroad wreathed in smiles, Trump is ending it with rebukes, upbraiding America's European partners over military spending, trade and global warming.
An enduring motif of the G7, which represents the lion's share of global economic output, has been to champion free trade.
At last year's summit in Japan, it issued a lengthy communique in support of resisting protectionism, as well as helping refugees and fighting climate change.
But that was then, when Barack Obama still occupied the White House. Today, his successor is defiant about stepping out of the G7 line.
"His basis for decision ultimately will be what's best for the United States," top economic advisor Gary Cohn said at the annual talks in Sicily.
Cohn was referring to whether Trump will execute his threat to walk away from the Paris accord on combatting climate change.
But his language also summarises the "America First" platform that elevated the property tycoon to victory in last year's presidential election.
That means the G7 is unlikely also to reprise its oft-used terminology against protectionism, after Trump in Brussels this week reportedly described the Germans as "bad, very bad" in their trade practices.
The club of leading democracies also looks set to fall short of last year's declaration on refugees and migration - the sort of language that is anathema to a White House that wants to impose a ban on travellers from six Muslim-majority countries.
It is a measure of the gulf that this year's Italian hosts say they expect the final statement to come in at just six pages when it is released on Saturday afternoon - down from 32 pages last year.
The summit did find common ground on Friday in endorsing a British call urging internet service providers and social media companies to crack down on jihadist content online, after 22 people were killed in a Manchester concert bombing in northwest England this week.
The G7, urged on by Japan, will also adopt common language against North Korea after a series of missile tests by the nuclear-armed nation.
Friday's discussions in Sicily ended with a classical music performance in the shadow of an ancient Greek theatre and the smoking volcano of Mount Etna.
Trump, however, seems bent on singing from a different song sheet, leaving the G7 bereft of a concerted voice as Russia and China are loudly heard offstage.
By Jitendra Joshi