Blair – among 450 world leaders to have signed the document – put his signature at a ceremony in Athens on Saturday undeterred by the fact that British soldiers were currently engaged in hostilities in Iraq.
"You cannot fight and play at the same time," said Stavros Lambrinidis, director of the International Olympic Truce Centre, underlining the significance of the initiative inspired by ceasefire between warring states during ancient games.
In addition to Blair, 20 prime ministers, heads of states and members of royal families signed the document during the day.
"You cannot fight and play at the same time"
Director of the International Olympic Truce Centre
Organisers said former US president George Bush Sr was invited, but declined to attend. German President Horst Kohler also signed the pledge to halt conflict during the sporting event.
The signatories added their names to a list that already included Pope John Paul II, UN Secretary General Kofi Annan and former US president Bill Clinton.
US not signatory
The truce is not observed by Washington, which has troops fighting in Afghanistan and Iraq.
Asked why US forces were not observing an Olympic ceasefire, US Secretary of State Colin Powell said that while the idea was a noble one, "the world does not stop entirely for the Olympic".
Blair made no comment and declined to talk to reporters after he signed the document.
The roots of both the truce and the Olympics dated back to the 9th century BCE. The idea was revived in 1993 to allow athletes from Yugoslavia to participate in the 1994 games.