The Prince and Princess of Wales have arrived in Boston, after a spokesperson for Prince William sought to distance him from a race row back in the UK.
William's godmother Lady Susan Hussey, a member of the Royal Household, resigned on Wednesday after repeatedly asking a black British charity boss where she was "really" from.
The royal couple are in the US to hand out awards for their Earthshot Prize.
They will also meet President Joe Biden, the White House has said.
The Earthshot Prize was launched in 2020 to search for innovative solutions to problems facing the planet.
Shortly after landing, the prince thanked the US city for hosting his environmental awards.
The royal couple launched the countdown to Friday's event by lighting up Boston's landmarks green.
Crowds stood in the rain to welcome William and Catherine to the city hall in Boston, home town of former US president John F Kennedy - the Earthshot prize is based on his 'Moonshot' challenge to put a human on the moon.
"Like President Kennedy, Catherine and I firmly believe that we all have it in ourselves to achieve great things, and that human beings have the ability to lead, innovate and problem-solve," Prince William told the crowd.
He said he wanted to thank the people of Boston for "their many tributes paid to the Queen", on this, his first overseas visit since her death.
"My grandmother was one of life's optimists. And so am I. That is why last year we launched the Earthshot Prize with the ambition to create a truly global platform to inspire hope and urgent optimism as we look to save the future of our planet," he said.
"To the people of Boston, thank you. I'm so grateful to you for allowing us to host the second year of the Earthshot Prize in your great city.
"Catherine and I can't wait to meet many of you in the days ahead."
Later on Wednesday evening, the royal couple attended an NBA basketball game between the Boston Celtics and Miami Heat at TD Garden, sitting courtside with Celtics owner Wyc Grousbeck and his wife Emilia Fazzalari.
A planned trip for the royal couple to New York earlier this year was cancelled following the death of the Queen. The couple was last in the US in 2014 in a trip that included a reception at the White House with the then president Barack Obama and his then vice-president Joe Biden.
When asked about the arrival of the royal couple in Boston, the White House said that President Biden intends to greet them and his team are "working through the details".
Five winners of the Earthshot Prize will be announced in Boston on Friday - each will be given £1m ($1.2m) to develop their environmental projects.
Among the 15 finalists are the inventors of a bubble pump designed to catch plastics before they reach the ocean.
For the first time there are also finalists from the UK, including London start-up Notpla, which makes packaging from seaweed and plants as an alternative to single-use plastic.
Prince William has described the people in the shortlist for the prize - now in its second year - as "innovators, leaders and visionaries".
The prize was inspired by US President Kennedy's "Moonshot" programme, which resulted in the US Apollo lunar launches and the first man setting foot on the Moon in 1969.
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