The country faces a shortage of troops in the campaign against Russia's better equipped and larger military
"I consider the accession of Sweden and Finland to NATO as an important signal of strengthening security in Europe," he said during a conference.
"I want to make it clear that in the event of an attack on Sweden or Finland during their accession (process), Poland will come to their aid."
Finland and Sweden formally applied to join the NATO alliance on Wednesday, a decision spurred by Russia's invasion of Ukraine, and setting in motion an accession process that is expected to take only a few weeks.