"I'm dressed like her, I smell like her, [and] just before I came out here I actually kissed a girl!"
The Wrecking Ball hitmaker - who describes herself as pansexual and is in a relationship with Liam Hemsworth - went on to joke she'd given Ellen the drug ecstasy, which is nicknamed Molly, to help cure her ailments.
She said: "Don't worry you guys Ellen will be fine, but she just isn't feeling well so she's asked me to fill in and of course, I said yes for my girl Ellen. And I gave her a bunch of Molly to make her feel better. It's working."
Miley was thankful the cheering crowd made her feel welcome because the outrageous star had felt "nervous" about stepping in.
She told the studio audience: "Thank you guys for making me feel so welcome.
"I'll admit I'm a little bit nervous because I've never hosted a show with my clothes on before so this is so weird!"
On the show, Miley went on to interview Sex and the City actress Sarah Jessica Parker and Frozen star Idina Menzel.
She also continued the regular host's Cat Week theme by airing a string of hilarious videos.