The couple let the public know where their allegiance lay, posting ‘Let’s go Celtics, let’s go!’ to their Instagram stories.
William and Catherine attended the match while visiting Boston, where they will attend the Earthshot Prize Awards Ceremony on Friday.
It’s the couple’s first visit to the United States in eight years, and their first overseas trip since the death of Queen Elizabeth.
It comes ahead of a meeting between the royals and US president Joe Biden, which the White House has announced will take place on Friday.
“The president intends to greet the Prince and Princess of Wales when he is in Boston. We are still finalising and working through the details,” White House Press Secretary Karine Jean-Pierre said at a briefing on Wednesday.
It’s the second year the Earthshot Prize is taking place. The global environmental prize aims to discover, celebrate and scale the best solutions to help repair our planet.
At this year’s Awards Ceremony, the Prince and Princess will join activists, innovators, policy makers and performers to celebrate the 2022 cohort of fifteen global Finalists before the five winners are awarded.