The president of the Czech Republic, Vaclav Klaus, has signed the EU's Lisbon Treaty, the final step in the charter's ratification.
The treaty was drawn up to streamline decision-making in the EU, and is a watered-down version of a draft EU constitution rejected four years ago.
Among its measures, it creates a European Council president and alters the way member states vote.
The treaty could now come into force as early as December.
The Lisbon Treaty's supporters say it will allow the EU to operate more efficiently and give it greater influence in world affairs.
Critics say it will cede too many national powers to Brussels.