Biden, a devout Catholic, lauded Benedict after his death for being "a renowned theologian, with a lifetime of devotion to the Church, guided by his principles and faith."
"I will always cherish our time together at the Vatican discussing theology," Biden wrote. "He was a great scholar of the faith."
The president, a Democrat, has been criticized by some in the Catholic Church for his support of abortion rights. Benedict's death was a loss for conservatives, who favored the more traditional Catholic Church that the former pope championed.
Biden signed the book at the Vatican's diplomatic mission in Washington. Vice President Kamala Harris also signed the book during a separate visit on Thursday evening.
(Reporting by Andrea Shalal, writing by Jeff Mason,; editing by Sandra Maler)
