The 'Firework' hitmaker - who has previously called the deceased Queen frontman her "biggest influence" - sang 'Happy Birthday' to the star and thanked him for helping her to become a "killer queen".
She said: "Happy 65th birthday Freddie, we love you so much. Even though you're gone, you're not really gone - you've inspired us all and you inspired me especially.
"Without you I wouldn't be making music or doing what I do because you're lyrics inspired me as a 15 year old girl. I heard 'Killer Queen' and I wanted to be a killer queen!
"Thank you so much for all of your amazing music, for Queen the band, for your whole persona, for your character and personality, which was larger than life. We love you and we'll never forget you."
Freddie died of bronchopneumonia brought on by AIDS on November 24 in 1991.
Katy - who is married to British comedian Russell Brand - has previously praised Freddie's "I don't give a f**k attitude" and admitted she almost got a giant tattoo of his face on her back as a tribute to him.
She said: "Freddie Mercury was - and remains - my biggest influence.
"The combination of his sarcastic approach to writing lyrics and his 'I don't give a f**k' attitude really inspired my music.
"I nearly got a huge tattoo of Freddie Mercury's face on my back a few years ago. It's probably for the best that I didn't get it done, but I still want to do it." Source
