Supreme Court Justice Sonia Sotomayor dropped by Clinton's book signing at a Costco warehouse in suburban Virginia, and surprised the former secretary of state.
Clinton shouted that she couldn't believe it.
Sotomayor told reporters she was "just shopping" and decided to get a signed copy of Clinton's new book, "Hard Choices."
The Saturday morning book event attracted more than 1,000 people, including Democratic Rep. John Lewis of Georgia, Costco co-founder Jim Sinegal and a few former Clinton aides and relatives.
The potential 2016 Democratic presidential candidate autographed copies for nearly three hours.
The former first lady kicked off her book tour this week, with stops in New York, Chicago and Philadelphia.