The Foreign Secretary told Tory members at a leadership hustings in Norwich on Thursday that she was undecided as to whether her counterpart in Paris was “friend or foe”.
A number of issues have affected the UK and France in recent months, including boat crossings in the Channel and travel chaos around Dover, which Ms Truss blamed on a lack of staffing by the French authorities.
Both the Foreign Secretary and her rival candidate Rishi Sunak were asked a series of quickfire questions at the Norwich hustings.
TalkTV’s Julia Hartley-Brewer, the event host, asked Ms Truss: “President Macron, friend or foe?”
“The jury’s out,” she responded to loud applause.
“But if I become prime minister, I would judge him on deeds, not words.”
The former chancellor had quickly answered “friend” when asked the same question.
Labour warned that the comment, which could be seen to risk straining tensions with France, showed a “terrible and worrying lack of judgment”.
Former Conservative minister Gavin Barwell also questioned the remark, tweeting: “You would have thought the Foreign Secretary was aware we are in a military alliance with France”.
Elsewhere in the hustings, Ms Truss conceded that if it were a choice between relying on France or China for nuclear expertise, she would pick France.
Taking questions in front of an audience of Tory members, she said: “I’m very clear that we need to boost our nuclear industry including Sizewell, including the small modular reactors that are produced in Derbyshire.
“Frankly, I would rather that we do have more homegrown nuclear expertise, and regrettably we lost that because we failed to do these things 20 years ago, or 30 years ago.
“If it’s a choice between relying on France and relying on China, I would take France.”
It comes after Ms Truss distanced the UK from the prospect of a project of being part of a wider European political community following a meeting between Boris Johnson and the French president in June.
The Elysee Palace insisted that the Prime Minister had expressed interest in the idea, which would see non-EU states such as the UK involved.
Ms Truss denied the UK had ever been on board with such a proposal, saying afterwards: “That is not true.
“I don’t know the exact words that President Macron has used, but we have not agreed to that.”
Asked whether she bought into “his political and economic community”, she replied: “No.”
In July, she said delays to the journeys of holidaymakers near Dover were the fault of French authorities and had been “entirely avoidable”.
Howeverm a French politician blamed Brexit for the chaos.
Pierre-Henri Dumont, Republican MP for Calais, said the problems at the Kent port would reoccur, telling BBC News: “This is an aftermath of Brexit. We have to run more checks than before.”