The Duke and Duchess of Cambridge arrived at their seats early in the second half of the Cleveland Cavaliers' game against the Brooklyn Nets on Monday night.
Cleveland's Kevin Love was shooting a free throw when fans stood and roared as the couple walked along the sideline to seats near center court.
LeBron James had called being watched by them as part of their first trip to New York "an honor" on Sunday.
The couple met earlier at Barclays Center with NBA Commissioner Adam Silver, celebrating a partnership between the Royal Foundation and the league to raise awareness about the importance of international wildlife conservation.
Crowds of officials began gathering near one of the tunnels to the arena floor at halftime in preparation of the royals' move to their seats.
They were shown on the overhead videoboard with about 2 minutes left in the period, getting another loud cheer when Kate waved to the camera.
After the third quarter, Jay-Z and Beyonce were led across the court to meet them. Jay-Z was formerly a part-owner of the Nets and has kept his seats next to the team's bench.
Before the game, Britain's Prince William went to Washington Monday to meet with President Obama about illegal wildlife trafficking at the World Bank, while his wife, Kate Middleton, toured a child development center with New York City's first lady, Chirlane McCray.
The Duke of Cambridge arrived early for his meeting with the president and was given a tour of the White House. He also met briefly with Vice President Joe Biden.
The White House said William was making his first trip to Washington. However, the president and wife Michelle have met William and Kate previously, including during a 2011 trip to London when the Obamas were hosted by Queen Elizabeth for a state dinner at Buckingham Palace.
On her second day in New York, Kate arrived at the Northside Center for Child Development in Harlem, a non-profit that provides education and health services to more than 3,000 families.
She and McCray were inside helping kids wrap gifts for Christmas.
Kate was greeted with cheers from a crowd of several dozen spectators standing behind police barricades across the street.
The royal couple checked into the Carlyle on Madison Avenue Sunday night. Their 3 day, first ever, visit to New York city is jam packed with events and fundraisers for causes close to their hearts.
"They cleared the lobby, so we came out here to see what we could see," said Steve Lipton, a tourist from Florida.
They were spotted out on the town Sunday night, attending a dinner in Gramercy. The Empire State Building is lit up in red, white and blue in their honor.
While the royals promote causes, many eyes will doubtless be on Kate's clothes. A style icon whose outfits can sell out in stores within hours after she's seen in them, she's won fashion-lovers' affection partly because she doesn't always choose complex or custom-made attire: "There's a connection - you can get it," or at least a copy of it, says Michael Casey, a professor at New York's Fashion Institute of Technology.
Sadly, Prince George did not come on the trip, so there will be no cute toddler sightings. This is a working visit for the couple, but for New Yorkers this is an exciting time to see the future of British monarchy.
Their visit also will include some events with other high-profile guests. Former Secretary of State Hillary Rodham Clinton and daughter Chelsea Clinton are accompanying William and Kate to a New York reception highlighting conservation efforts.
William and Kate also are due to join Tom Hanks, opera singer Renee Fleming and others at a black-tie, up-to-$10,000-per-seat scholarship fundraiser for the University of St. Andrews, the Scottish institution where the royal couple met and earned degrees.
(Some information from the Associated Press)