The 28-year-old wore a skin-tight latex number which depicted a ballot card – and clearly filled in on the outfit was the space close to the names of Mr Obama and Vice President John Biden. Underneath, the box corresponding to Mr Obama's rival Mitt Romney was left blank.
Katy opened her set with one of the president's favourite tunes, Al Green classic Let's Stay Together, and performed five songs – including her hits Teenage Dream and Firework. Behind her, images of the US leader – and his slogan 'Forward' were beamed onto a giant screen.
Katy took the opportunity to urge the crowd – which numbered over 13,000 people – to go vote early. "Don't wait," she implored. "How many of you guys are 18 in here? This is your first time, right? Let's do this tomorrow. You know, I've got my poll outfit sorted – when I go to the polls in California it's going to be on point. It's going to be red white and blue for you know who."
There were huge cheers when the man of the moment took to the stage. He told his supporters: "I am so grateful to Katy Perry for that unbelievable performance. I've been getting to know Katy, she's just a wonderful young lady.
"And I can tell that part of it is because of her outstanding grandma Anne, a 50-year resident of Nevada. The only thing I have to say is Anne got some lipstick on me when she kissed me. Fortunately somebody wiped it off before Michelle saw it."
He continued: "Nevada, you know me by now. You know I say what I mean, and I mean what I say. We haven't finished everything we set out to do in 2008, but you know that every single day I set foot in that office, I am thinking about you. I'm fighting for your families. And with your help, I've been keeping the commitments that I made."
En the way to the airport, he made a surprise visit to the Bellagio hotel and casino, where hundreds of excited workers received him.
He told them: "Everyone here knows what it means to work hard. You're what makes Las Vegas run. You’re what makes the strip run. I need you guys to go out and vote now. Don’t wait to vote. You're going to go cash in your chips now."
Mr Obama is currently polling a few points ahead in Nevada, the state which he comfortably won in the 2008 presidential rally. But the huge crowd that turned up for his rally this week easily outnumbered the 5,000 who turned up for a similar event for Mr Romney in Las Vegas a few days ago.