The 'Orange Is The New Black' star took to social media to criticise the 'Chained To The Rhythm' hitmaker's new song, which she dubbed a "sloppy mess".
She wrote on her Twitter account:
"Purposeful poop" to "bomb a petit" to a sloppy mess of writing over the top of Funkagenda..stop trying to make 'Wit..I mean "fetch" happen.
"I just think with everything going on in the world to go from rebranding as political activist only to ditch it and go low.. is.. a bummer. (sic)"
Katy teamed up with Nicki Minaj on the song and Ruby went on to praise Katy's collaborator, who she says did the "only part that doesn't sound a mess".
She added: "Not gonna lie Nicki is great on it and basically the only part that doesn't sound a mess (sic)"
The 32-year-old singer previously revealed she has launched a new "purposeful pop era".
She wrote on Twitter at the time: "We gonna call this era Purposeful Pop ... question everything ... Evolution right on track (sic)"
And Katy previously called for women to "come together" to make the world a "better place".
She said: "Honestly, when women come together and they decide to unite, this world is going to be a better place. Period end of story ...
"I think [my new album is] a very empowered record. There is no one thing that's calling out any one person. One thing to note is: You can't mistake kindness for weakness and don't come for me. Anyone. Anyone. Anyone. Anyone. And that's not to any one person, and don't quote me that it is, because it's not. It's not about that.
"This record is not about anyone else. This record is about me being seen and heard so that I can see and hear everyone else. It's not even about me! It's about everything that I see out there that I digest. I think there's a healing in it for me and vulnerability. If people want to connect and be healed and feel vulnerable and feel empowered and strong, God bless, and here it is."