The actor cried "a tear of joy" when his wife-to-be belted out her mega-hit "Firework" during the TV special in honour of President Joe Biden's inauguration on Wednesday (20Jan21).
And Orlando can't wait for the couple's four-month-old daughter, Daisy, to be old enough to see her mother "play (her) part in a moment in history."
Alongside a clip of his fiancee's powerful performance of her 2010 hit, Orlando wrote, "One day our little girl will grow up and see her mother play her part in a moment in history that we hope will heal, we hope will unite, we hope, we love."
"One proud partner here with a tear of joy."
Katy belted out "Firework" at the National Mall as the night sky was lit up behind her with an incredible firework display, with the performance quickly leading to her trending on Twitter.
She later shared a clip of the performance on her Instagram page, writing alongside it, "Firework - Celebrating America."
Katy's "Firework" performance closed out the TV special which kicked off with Bruce Springsteen's powerful rendition of "Land of Hope & Dreams." The Boss said, "I'm proud to be here in cold Washington, D.C."
Tom Hanks served as a host, "Tonight, we ponder the United States of America, the practice of democracy, the foundations of a republic, the integrity of our Constitution, the hopes and dreams we all share for a more perfect union."