Prince Harry was accompanied into St George's Chapel at Windsor Castle with older brother Prince William by his side.
The two royals, dressed in their military uniforms, looked cheerful and excited as they entered the chapel.
Kensington Palace released a statement that reads: 'Both Prince Harry and The Duke of Cambridge will wear the frockcoat uniform of the Blues and Royals.
'Her Majesty The Queen gave her permission for Prince Harry to get married in his uniform.
'Both uniforms were tailored at Dege & Skinner on Savile Row.'
It was announced in April that Prince Harry had officially asked Prince William to be his best man at his wedding to Meghan Markle on May 19.
"Prince Harry has asked his brother The Duke of Cambridge to be his Best Man at his wedding to Ms. Meghan Markle," a Tweet from the palace confirmed at the time.
"The Duke of Cambridge is honoured to have been asked, and is very much looking forward to supporting his brother at St George's Chapel, Windsor on May 19th."
Prince Harry served as best man for William at his wedding to Kate Middleton in 2011.