Kate, the duchess of Cambridge, laid a bouquet of white roses and a handwritten note at one of the two memorial pools marking where the World Trade Center towers once stood.
Etched along the pools are names of the nearly 3,000 people killed on Sept. 11, 2001, when two hijacked planes destroyed the twin towers.
The note, signed by the couple, read: "In sorrowful memory of those who died on 11th September, and in admiration of the courage shown to rebuild."
The couple, who are expecting their second child in April, are making their first visit to New York City.
At the memorial, they waved briefly to bystanders before heading inside to tour the museum exhibits of artifacts and photographs from the day of the attacks.
Kate wore a fuchsia Mulberry coat that stood out brightly in the gray of a rainy day but hid her baby bump.
Huddled under an umbrella was Ben Jones of Leeds, England, who said he and his wife had planned a trip to the memorial and were quite surprised to find their country's royals there at the same time.
"It just so happened that we turned up," he said. "I've never seen these two so we might as well hang about."
New Yorker Susan Daglian said she missed every other royal visit to the city and vowed not to miss this one, despite the weather.
"Just to get a glimpse is worth it," she said. "I think they're wonderful."
The visit, which began on Sunday, ends on Tuesday with a dinner marking the 600th anniversary of the University of St. Andrews at the Metropolitan Museum of Art.
On Monday, William and Kate attended a professional basketball game in Brooklyn. Outside the arena, demonstrators gathered in what have become nightly protests over a grand jury decision not to indict a white New York City police officer in the chokehold death of an unarmed black man.
