The 35-year-old pop star suffered depression during her Witness tour in 2017 and 2018, and Katy has now revealed she took medication in order to fight the problem.
She shared: "I was on the Witness tour, I remember looking out to the audience and thinking, 'Why are you here? You don't like me. I'm not that good.' I got very clinically depressed."
Katy has been in therapy since she was 25, but her most-recent encounter with depression was more difficult than anything she'd previously experienced.
The 'Roar' hitmaker said: "I had always been able to skirt the issue.
"Any time I suffered from bouts of depression I would easily be able to run to the validation of the outside world by writing a song, doing a music video, getting likes and comments [on social media]."
Katy admits to taking medication to combat the problem - although she hasn't specified what it was.
She told the Sunday Times newspaper: "I tried medication and that was really intense. I was on something that my psychologist at the time recommended.
"It was like I sprained my brain and I needed crutches. It changes the chemistry of your mind, all the serotonin and dopamine. Sometimes people need a pharmaceutical crutch."
Katy is no longer on medication, but she is a fan of meditation and the Hoffman Process, which is an extreme course of therapy.
She said: "You know that voice in your brain that says, 'You're fat, you're ugly, you just got lucky,' all that s***? It silences it.
"Which is great because mine was super-loud."