"Only Ukraine has the right to decide its future", the Polish president has said in a speech to the Ukrainian parliament.
Andrzej Duda said "worrying voices" were suggesting Ukraine should give in to Russia's President Putin, according to comments quoted by the Reuters news agency.
He told lawmakers that those voices should not be heeded - because ceding even an inch of Ukrainian territory would be a blow to the whole West.
Duda also reportedly stressed that Poland supported Ukraine in its bid to gain full membership of the European Union - a matter on which EU countries have not reached a consensus.
He was the first foreign leader to give an in-person speech to the Ukrainian parliament since Russia invaded on 24 February.
Photos posted on Twitter by Duda's office showed him receiving a standing ovation in parliament and embracing Zelensky.
