Prince William, Kate Middleton watch courtside as Boston Celtics take on Miami Heat during their US trip

The Prince and Princess of Wales attended a Boston Celtics home game vs. the Miami Heat during the royals' first trip abroad since Queen Elizabeth II’s death in September and their first trip to the U.S. in eight years.

Prince William and Kate Middleton, both 40, walked into TD Garden in Boston while flanked by security shortly before the game began on Wednesday evening.

Middleton, who was clad in a royal blue blazer, stood out in a sea of green as she strolled alongside her husband to their courtside seats. 

William color-coordinated with Middleton in a bright-blue button-down shirt underneath a navy blue blazer.

PRINCE WILLIAM, KATE MIDDLETON HAVE ‘NO PLANS’ TO SEE MEGHAN MARKLE, PRINCE HARRY DURING US TRIP: SOURCE

The couple joined Celtics co-owner Wyc Grousbeck and his wife, Emilia Fazzalari, co-owner Steve Pagliuca and his wife, Judy, Celtics great and Hall of Famer Thomas "Satch" Sanders, Massachusetts Gov.-elect Maura Healy and Boston Mayor Michelle Wu on the sidelines. During the game, Middleton was seen chatting with Fazzalari and speaking to attendees who were seated behind her.

Celtic players were seen walking by William and Kate as they checked in for the game while head coach Joe Mazzulla occasionally paced in front of the couple.

During the game, the Heroes Among Us award was presented to Ollie Perraul, a 15-year-old climate activist from Easthampton, Mass. The royals were seen talking with Perraul after the award presentation.

The couple also met with members of the Celtics family, including representatives of the Boston Celtics Shamrock Foundation, which provides grassroots programming and strategic funding to local organizations serving at risk or at need populations.

William and Kate received mixed reactions when they were shown on the jumbotron as some fans applauded and cheered, while others appeared to boo the royals. However, the pair appeared to be enjoying themselves and William was spotted clapping and cheering the players on throughout the game.

Ahead of the game, the couple's official Twitter account shared a post, writing, "Let's go Celtics, let's go!"

William and Kate's Wednesday night appearance was not the first time that the couple have attended a professional basketball game in the U.S. During their last stateside trip in 2014, the couple enjoyed a Brooklyn Nets game in New York City.

Earlier in the day, the two arrived at Boston's Logan International Airport before 3 p.m. ET. They traveled to Boston for William’s Earthshot Prize, which awards grants to people working in environmentalism. Promos for the Earthshot Prize were flashed on the jumbotron throughout the Celtics game.

"Catherine and I are delighted to be back in the United States and are extremely grateful to Governor Baker and The First Lady of Massachusetts for their warm welcome into Boston," William said in a statement after their arrival.

He added, "On this, our first overseas visit since the death of my grandmother, I would like to thank the people of Massachusetts and particularly of Boston for their many tributes paid to the late Queen."

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William said the queen remembered her 1976 visit to the city during the bicentennial "with great fondness."

He continued, "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. 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."

The couple flew into the city on British Airways and will be in Boston through Friday, when they will announce the Earthshot Prize winners.

President Joe Biden plans to meet with the royal couple this week, according to White House press secretary Karine Jean-Pierre, who said, "The president intends to greet the Prince and Princess of Wales when he is in Boston. We are still finalizing and working through the details, so I don't have any anything more to share, any more specifics to share on that."

During their trip, Wu will host the couple at Boston Harbor for a tour of the environmental challenges and rising sea levels facing the East Boston shoreline on Thursday. They will meet with Ambassador Caroline Kennedy at the John F. Kennedy Presidential Library on Friday.

Kennedy's Moonshot that challenged NASA to put a man on the moon in the early 1960s was the inspiration for the Earthshot Prize's name. 

Later Friday, the three-day visit will culminate in William and Kate announcing the winners of the Earthshot Prize grants at MGM Music Hall at Fenway. The ceremony will be headlined by Billie Eilish, Annie Lennox, Ellie Goulding and Chloe x Halle will also perform.

Winners in five categories, including nature protection, clean air, ocean revival, waste elimination and climate change will receive a 1 million pound grant ($1.2 million) toward furthering their work. All 15 finalists will receive financial help.

Fox News Digital's Brie Stimson contributed to this report.

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