The Fireworks hitmaker has been dating Mayer since last year (12) and they've recently taken their relationship to the next level by house-hunting in California.
But, instead of walking the red carpet as a couple, Perry has invited Girls star Williams, whose boyfriend Ricky Van Veen is close friends with Mayer, to accompany her to the big show on Sunday (10Feb13).
Williams tells U.S. TV host Anderson Cooper, "I am going to the Grammys. Katy Perry is my date. She is the hottest date. She's a friend of mine, and she's nominated, as she should be, and I can't wait to go. She's awesome. She's the best."
The news comes just two weeks after Mayer was pictured out in New York City with the actress, sparking rumours of a blossoming romance. It later emerged both stars were longtime friends.