Trump, who enjoyed breaking with political convention on his way to the White House, likes ruffling political feathers, and also enjoys predicting further political earthquakes around the globe that echo his shock November win.
In comments to the Associated Press, he said he was not endorsing Le Pen, but nevertheless called her the candidate who is "strongest on borders, and she's the strongest on what's been going on in France."
The Republican president predicted the attack claimed by the ISIS group, in which a gunman shot dead a policeman and wounded two others on the world-famous Champs-Elysees avenue, would "probably help" her chances.
"Another terrorist attack in Paris. The people of France will not take much more of this. Will have a big effect on presidential election!" he tweeted earlier in the day.
Although the ISIS very quickly claimed responsibility, it wrongly identified the attacker, raising questions about its links to the incident, or at the very least to the shooter.
Police did however find a handwritten note praising ISIS near the slain attacker.
Whatever its provenance, the attack rocked France's presidential race with just days to go before the first round of voting in one of the closest contests in recent memory.
The country is in a state of emergency and at its highest possible level of alert since a string of terror attacks that began in 2015, which have killed more than 230 people.
Bloodshed had long been feared ahead of Sunday's first round after a string of jihadist atrocities since 2015, and the shooting on the Champs-Elysees forced security to the top of the agenda in the campaign.
Backing Le Pen?
Three of the four presidential frontrunners -- Le Pen, centrist Emmanuel Macron and conservative Francois Fillon -- called off campaign events planned for Friday in the wake of the attack.
Le Pen was one of the first to offer Trump her congratulations.
But when she was spotted in Trump Tower in January, Trump's team made it clear that no meetings were scheduled.
And despite complimentary comments about some of her policies, White House spokesman Sean Spicer firmly denied that Trump had a preferred candidate in the race.
The US president has portrayed himself as a wily prognosticator of European politics more generally, notably claiming he foresaw Brexit. He also says he believes other countries will follow London's example.
Beyond France, he has portrayed his victory as the start of a new political era, one in which more electoral surprises are to come.
Macron, a 39-year-old moderate, is presenting himself as a guarantor of the EU, promising reform and appealing to pro-Europe sentiment among the French electorate.
Le Pen, widely seen as taking the hardest line on security, has called for France to "immediately" take back control of its own borders from the European Union and deport all foreigners on a terror watchlist.
Barack Obama spoke to Macron on Thursday about the "important upcoming presidential election in France," a spokesman for the former US president said.
Other US presidents have weighed in -- some directly, others more obliquely -- on foreign elections in the past.
In 1996, Bill Clinton made it clear he preferred Labor candidate Shimon Peres over Likud contender Benjamin Netanyahu. But his efforts were for naught, as Netanyahu won the election.