The Portuguese rotation of around 150 troops will come from the Portuguese army's marine corps, Lithuania's Ministry of National Defense said.
During the rotation period, the Portuguese troops will also train with the Lithuanian Navy and Special Operations Forces.
The Portuguese troops are arriving in Lithuania in groups from May, with the main contingent set to come on Tuesday. Meanwhile, around 40 units of military equipment will be brought to the port of Klaipėda on Wednesday.
Portugal is sending troops to Lithuania under NATO's deterrence measures in Central and Eastern European countries, aimed at strengthening allies' security.
Troops from Belgium, the Czech Republic,, Denmark, the US, Poland, Luxembourg, Hungary and Germany have come to Lithuania since 2014.