Thalys said in a statement Tuesday that travellers booked on trains running between Belgium and Germany could exchange their tickets or ask for full reimbursement.
Ticket-holders planning to travel to other destinations Wednesday should check thalys.com for possible disruptions due to the protests.
Trade unions across Europe have made Wednesday a day of action against austerity.
In Belgium, rail traffic is expected to be hardest hit after trade unions called for protests in different sectors throughout the day.
In Spain and bailed-out neighbour Portugal, the main unions have called general strikes which threaten to bring both countries to a standstill after massive protests earlier in the year.
Greece, the epicentre of the debt crisis, will see another day of demonstrations while French unions plan action too on a day of solidarity "with the countries in greatest difficulty".
Smaller protests are expected in Italy.