The Prince and Princess of Wales will hand out awards for their Earthshot Prize on Friday evening.
Image source, PA MediaThe Prince of Wales will meet US President Joe Biden later on the final day of his US trip.
Prince William will be greeted by Mr Biden on a visit to the John F Kennedy Presidential Library and Museum.
In the evening, William and the Princess of Wales will hand out awards for their Earthshot Prize – given to innovative ideas for the environment.
However, the high-profile trip has been overshadowed by a racism row involving William’s godmother.
Lady Susan Hussey, a key member of the Royal Household, resigned after repeatedly asking a black British charity boss where she was “really” from.
Prince William’s spokesperson has said “racism has no place in our society”.
This is the royal couple’s first big international appearance since the death of the Queen, and since they became the Prince and Princess of Wales.
Image source, PA Media
Image source, PA Media
They will hand out awards to five Earthshot Prize winners during a ceremony in Boston in the evening. Each winner will be given £1m ($1.2m) to develop their environmental projects.
The awards ceremony will be a mix of good causes and entertainment – performers include Billie Eilish, Annie Lennox, Ellie Goulding and Chloe x Halle.
Ahead of the event, William and Catherine have been pictured behind the scenes during rehearsals.
Viewers will have to wait to see the awards ceremony – it is due to be broadcast at 17:30 GMT on Sunday on the BBC in the UK, and on Monday in the US.
Prince William’s brother Harry, the Duke of Sussex, and his wife Meghan are not expected to attend the awards ceremony.
On the second day of Prince William and Catherine’s trip, the trailer for Harry and Meghan’s “behind closed doors” Netflix series was released.
The minute-long clip features previously unseen pictures of the Duke and Duchess of Sussex, including tearful images of Meghan.
The BBC’s Daniela Relph, who is in Boston, says not to expect the Prince and Princess of Wales to comment on the Netflix series during their US trip.
Among the 15 finalists for the Earthshot Prize 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 said the Earth is at a “tipping point” and the awards are intended to support innovative ways of reducing environmental damage.
The prize was inspired by US President John F Kennedy’s “Moonshot” programme, which resulted in the US Apollo lunar launches and the first man setting foot on the Moon in 1969.