The 35-hour week was introduced by a previous Socialist-led government in 2000 in a bid to redistribute work and create jobs and is fiercely protected by the French left - despite the fact that many French in reality work much longer hours than that.
In an interview with weekly Le Point just before his appointment on Tuesday but only released on Thursday, the former banker backed further labour market reform because "that does not have a deflationary impact and could restore confidence".
"We could authorise companies and sectors, provided there is a majority (union) agreement on this, to have exceptions to the rules on working time and remuneration," he added.
His words drew immediate criticism from Laurent Berger, head of the France's moderate CFDT trade union, who insisted Macron had "made a mistake".
"It's out of the question. The subject is closed," he told i>Tele television.
President Francois Hollande's government has introduced modest reforms to the labour market but stayed well clear of changing the 35-hour working week. His government has suggested new reforms could be envisaged, for example in adapting costly requirements on companies to set up worker councils.
The 36-year-old former presidential adviser replaced leftist Arnaud Montebourg as economy minister in a government reshuffle this week, to the consternation of many on the left wing of the Socialist Party.
Montebourg was pushed out of office by President Francois Hollande for his outspoken opposition to public spending cuts aimed at bringing France's public deficit down within EU limits.