German Foreign Minister Frank-Walter Steinmeier said he was "delighted" that the Polish had signed the EU's reform treaty, describing Warsaw's move as the "penultimate step" toward the treaty's implementation.
Top European diplomats joined in the chorus of praising the Polish signing of the Lisbon Treaty.
EU Commission President José Manuel Barroso, who attended Saturday's signing ceremony in Warsaw, described the move as a "very important chapter for Poland and the European Union."
Polish President Lech Kaczynski said the EU was "a great experiment in the history of mankind," which would "function even more effectively" when the treaty takes effect.
He signed the treaty only after Ireland ratified it last week with an overwhelming "yes" vote in the second referendum on the subject.
The euroskeptic Polish president had delayed signing the document as long as the Irish vote was undecided, saying that the smaller states in the EU shouldn't be coerced by the larger members of the 27 nation union.
"A union of sovereign nation states"
Before he signed the document, President Kaczynski stressed that "the European Union remains a union of sovereign nation states and let it be so."
"We now have 27 member states. I am deeply convinced this is not the end... The EU, a successful experiment without precedent in human history, cannot be closed to those who wish to join... not only in the Balkans but also countries like Georgia."
The Lisbon Treaty is designed to give the 27-nation bloc a long-term president and a stronger foreign policy chief. It can only take effect when all member states have approved it.
Poland's signature leaves the Czech Republic as the sole remaining hold-out on the Lisbon Treaty, as its own euroskeptical President Vaclav Klaus awaits the outcome of a legal challenge of the treaty.
The Charter of Fundamental Rights within the Lisbon Treaty, Klaus says, could allow claimants "to bypass Czech courts and enforce the property claims of people expelled after World War II at the European Court of Justice."
Any further delay would be disappointing to Barroso who hoped to see the treaty take effect by January 1, 2010.