"Whose broad stripes and bright stars?" Katie captioned the pic which shows the pair in matching patriotic shirts in his tour bus, which was parked backstage.
She also Tweeted a pic of her red, white and blue nails.
Katy was backstage to watch her man perform, and appeared to give the "California Gurls" singer a shout-out by performing one of his lesser-known songs, "Queen of California."
Fans at the concert Tweeted about seeing the pair being affectionate with one another with one eyewitness Tweeting that John and Katy were "definitely holding hands backstage."