Stevedores at Finnish ports halted loading and unloading on Thursday at 6 a.m. (0400 GMT).
"Port negotiations ceased just after midnight. A new time for talks was not set. The strike begins at ports at 6 a.m.," the union said in a statement.
The talks failed over union demands for redundancy pay for dockers equivalent to one-year pay, which employers reject as unreasonable.
On Wednesday the union struck a deal on wages with road transport employers, allowing bus and goods traffic to resume after a strike started on Tuesday evening.
The stoppage at ports will hurt many industries in export-dependent Finland -- as it makes a slow recovery from the downturn -- including forestry, electronics and metals.
The Confederation of Finnish Industries, EK, told Finnish broadcaster YLE on Wednesday the strike would cost exporters around 100 million euros ($137 million) per day.
Paper firms UPM-Kymmene and Stora Enso said separately the strike would force them to close paper mills within days. UPM said each day its machines were not running would cost it more than 3 million euros.