Kensington Palace officially made the announcement on Thursday!
“The Duke of Cambridge is honored to have been asked, and is very much looking forward to supporting his brother at St. George’s Chapel,” reads the statement released by Kensington Palace on Thursday.
Prince Harry, of course, served as his big brother’s best man in his and Kate Middleton's 2011 royal wedding at Westminster Abbey.
The news comes just days after William welcomed his third child with the Duchess of Cambridge, and while the couple introduced their newborn son to the public on Monday, they've yet to reveal his name.
William, 35, and Harry, 33, both attended the Anzac Day Service of Commemoration and Thanksgiving on Wednesday, alongside Markle. William was photographed sweetly greeting his future sister-in-law outside of Westminster Abbey with a hug and kiss on the cheek.
William was also overheard saying the new baby was doing “very well” and added, “Sleeping’s going reasonably well so far, so he’s behaving himself, which is good.”
It's not yet known if the newborn baby will attend Harry and Meghan's nuptials, but it was confirmed earlier this year that William and Kate's son and daughter, Prince George and Princess Charlotte, will have roles in the wedding. George, 4, will be a page boy, while Charlotte, 2, will serve as a flower girl.
Keep up with all of ET's coverage of the Royal Wedding right here as we count down to May 19!
Prince William Joins Prince Harry and Meghan Markle for Anzac Day Service Following Birth of Royal Baby