"From May 4, passengers must wear a face mask or face covering at our stations and on board in line with guidelines announced by the French and Belgian governments," the company said on its website.
It warned passengers that anyone not covering their mouth and nose may be refused travel or fined by the French and Belgian authorities.
UK-based Eurostar has been running a reduced timetable since the outbreak, because of movement restrictions, enhanced border checks and lower demand.
Important update: From 4 May, passengers must wear a face mask or face covering in our stations and on board, in line with guidelines from the French and Belgian governments. Find out more here:https://t.co/mahsEnm8Hg.
— Eurostar (@Eurostar) May 2, 2020
Catering facilities on trains and in stations have been closed, and special assistance suspended because of social distancing requirements, the company said on its website.
The measures are among several to be introduced as governments on both sides of the Channel look at easing stringent lockdown restrictions.
France
It also advised passengers that France was restricting travel to EU citizens of mainland continental Europe returning home to their main residence.
UK and non-EU citizens with a residency permit in an EU country, plus essential workers, were allowed to travel, it added.
But all passengers required additional paperwork in France to show their trip was essential.
France is gradually lifting its lockdown restrictions from May 11.
Belgium
Belgium, which has had the highest per capita rate in the world -- 67 per 100,000 people -- begins to ease its lockdown on Monday.
Anyone travelling on public transport in Belgium is required to cover their mouth and nose or risk a fine.
Britain
Britain has not told the public to wear masks, although Prime Minister Boris Johnson this week indicated they could be "useful" when restrictions are lifted.
(with wires)
