Obama, who was touring the church Tuesday with first lady Michelle Obama, signed his name with the date "24 May 2008," instead of the current year, the Canadian Broadcasting Corp. reported Wednesday.
"It is a great privilege to commemorate our common heritage, and common sacrifice," the Obamas wrote in the guest book.
A Westminster Abbey spokesman confirmed it was President Obama who signed the incorrect date.
President Obama also placed a wreath at the Grave of the Unknown Warrior at the church, The Daily Telegraph reported. He was given a tour of Westminster Abbey from the Very Rev. Dr. John Hall.
"Westminster Abbey is known as the House of God and the House of Kings, where God is worshiped every day and where many great men and women are buried or memorialized," Hall told the Obamas.
"Here has taken place every coronation of kings and queens of England and of the United Kingdom since William I was crowned here on Christmas Day 1066."
Hall noted thousands of international visitors "including very many from the United States are welcomed" every day.