Andrzej Duda's move was announced on social media by his security policy aide, Paweł Soloch.
The president made the decision in coordination with the country's defence ministry and in line with a pledge he made at a NATO summit in June, according to Soloch.
Ratification of the Eurocorps treaty is set to "make Poland a framework nation, alongside France, Germany, Spain, Belgium, and Luxembourg," Soloch said on Wednesday.
Bigger role for Poland in Eurocorps
An associated nation since 2002, Poland will now become a full-fledged member, with a say over the composition and tasks of the multinational group, public broadcaster Polish Radio's IAR news agency reported.
Based in Strasbourg, France, the 1,000-strong Eurocorps is a rapid-response military force capable of commanding up to 60,000 soldiers. It can be used in humanitarian, rescue and peacekeeping missions by international organisations such as the European Union, NATO and the United Nations.
As a high-readiness NATO force, the Eurocorps can also participate in the alliance's collective defence operations under Article 5 of the NATO charter. It is led on a rotating basis by commanders named by participating countries.