The 23-year-old openly bisexual singer has also admitted to having a crush on 'fit' Katy.
She told British More! magazine: 'Who's hotter out of Russell and Katy? Katy! She's so fit, and has the most amazing face and boobs.'
Perry, 26, is married to 36-year-old TV presenter Russell Brand.
The Price Tag hitmaker also said she's not looking for a relationship at the moment and is instead focusing on 'loving herself'.
Jessie added: 'How did I know that question was coming?! I've got to focus on loving me right now. Love can be a very powerful thing and I think I have to live my life how I want to live it.
'There are some things I've learnt you have to keep private for your sanity's sake.'