Klaus was an opponent of the treaty until the end, Financial Times reported, only signing the document hours after the Czech constitutional court ruled the agreement did not violate the country's Constitution.
Even at the last minute, Klaus called the court ruling biased, intoning "the Czech Republic will stop being a sovereign state" before announcing he had signed the treaty at 3 p.m. local time, the British business newspaper reported.
Klaus and other opponents contend the treaty will lead to tighter European integration and too much power in Brussels, the Times said. The treaty had been approved by the Czech Parliament last year.
The court challenge was brought by conservative lawmakers who argued the treaty violated national laws, the EUobserver reported.
Pavel Rychetsky, the court's chief justice, told state-run TV he had determined the treaty "is not in conflict with the constitutional law of the Czech Republic."
The country is the last EU member state to fully ratify the treaty, which must have the approval of all members to put its provisions into effect. The process had been held up by Klaus, a strong skeptic of the European Union.
The EUobserver said Klaus backed down from his opposition last week by offering to sign the accord if the Czech Republic was given the power to opt out of the requirements of the treaty's human rights charter.