Stevedores are due to suspend work at 07:00 a.m. Brasilia time for a full day but could extend the strike indefinitely, union President Rodnei Oliveira da Silva said.
"All terminals will be affected," he said.
It is still unclear what types of cargo will be affected. Typically dry bulk loading of soybeans, raw sugar and corn continues uninterrupted during stevedore strikes, as manual labor needs are limited for those operations.
But loading of containers and other more labor-intensive goods could suffer.
Brazil's record soybean harvest is about to peak and record sugar and coffee harvests are about to pick up in the coming weeks to months.