ISIS claimed responsibility for the fatal shooting late on Thursday of a French policeman on the Champs Elysees by a French national who lived in Paris.
"Another terrorist attack in Paris. The people of France will not take much more of this. Will have a big effect on presidential election!" Trump said on Twitter.
The attack prompted far-right presidential candidate Marine Le Pen to say on Friday that France should reinstate border checks and expel foreigners who are on the watch lists of intelligence services.
Le Pen, who has campaigned on an anti-European Union, anti-immigration platform, was the only major French candidate who backed Republican Trump in the Nov. 8 U.S. presidential election.
In his election campaign, Trump seized on the last year's Brexit vote in the United Kingdom as an example of disillusioned voters rising up against the political establishment and forged a friendship with Nigel Farage, a leading campaigner for Britain's withdrawal from the EU.
On Thursday, former US President Barack Obama spoke with a different French candidate, Emmanuel Macron, a pro-EU centrist.
Macron is leading most opinion polls for the election's first round on Sunday and is expected to contest a second-round run-off with Le Pen. Obama's spokesman said the former US president, who is popular in France, was not making a formal endorsement.