Kate Middleton and Prince William dazzle at Earthshot Prize ceremony in Boston

Prince William and Catherine, Princess of Wales, stepped out in style to close out their three-day trip to the United States.

Prince William and the former Kate Middleton attended The Earthshot Prize at MGM Music Hall at Fenway on Dec. 2 in Boston, Massachusetts.

The Princess of Wales looked regal in an off-the-shoulder bright green gown by designer Solace London. A Kensington Palace spokesperson told NBC News that the former Kate Middleton borrowed the dress from the U.K. rental platform HURR.

The 40-year-old royal paired her glam look with an emerald and diamond necklace that once belonged to the late Princess Diana, matching earrings and silver heels.

Prince William, on his end, looked dapper in a deep navy velvet tuxedo jacket with black slacks.

The royal couple waved at the crowd who gathered around the event and green carpet, and posed for photos as they made their way indoors to the event.

The Earthshot Prize is awarded to five winners each year for their contributions to environmentalism. It was created in 2020 by Prince William and David Attenborough.

“I believe that the Earthshot solutions you have seen this evening prove we can overcome our planet’s greatest challenges. And by supporting and scaling them we can change our future,” the Prince said during the event, per a press release. “Alongside tonight’s winners and finalists, and those to be discovered over the years to come, it’s my hope the Earthshot legacy will continue to grow, helping our communities and our planet to thrive.”

Additional guests included Billie Eilish, singer-songwriter Annie Lennox, actors Rami Malek, Daniel Dae Kim and Shailene Woodley, as well as Senator Mitt Romney and wife Ann Romney, Boston Celtics owner Wyc Grousbeck and wife Emilia Fazzalari, and John Kerry, who serves as the first U.S. special presidential envoy for climate.

David Beckham made an appearance at the ceremony. The former British soccer player wore a black tuxedo for the formal event.

British singer Ellie Goulding also showed her support, wearing a white billowing gown.

“Schitt’s Creek” star Catherine O’Hara and husband Bo Welch posed for photos as they arrived to The Earthshot Prize ceremony.

The actor wore a floral suit, while the production designer wore a black suit with a gray coat.

Sister and singing duo Chloe and Halle also made a stylish appearance. Chloe stunned in a strapless navy gown with a full skirt, which she paired with black boots and gloves.

The “Little Mermaid” star, on her end, looked radiant in a lime green one-shoulder dress.

The Prince and Princess of Wales' appearance came hours after William met with President Joe Biden at the John F. Kennedy Presidential Library and Museum. While at the establishment, he also met with Caroline Kennedy.

The former Kate Middleton was not present, as she visited The Center on the Developing Child at Harvard University.

The British royals arrived to the city on Nov. 30. Among visiting various organizations, they also sat courtside at the Boston Celtics NBA game against the Miami Heat.

The couple is expected to return home following The Earthshot Prize event.

This article was originally published on TODAY.com