The pair were seen Saturday at Britain's Glastonbury rock festival, where they took in the Rolling Stones show together.
Though Harry, 28, managed to duck photographers, Bonas, 24, was spotted in distressed denim overalls, white Nikes and neon green sunglasses - at one point with a drink in her hand and, later, a plate of fries.
The festival's organizer, Michael Eavis, told The Telegraph that after the Stones concert ended at midnight, he recommended that the couple come back later to take in some of the local nightlife.
"I told him to get his taxi driver to come back at five o'clock in the morning, and do you know what? He lasted until four in the morning."
"His friends were all having a great time. He didn't want to make a formal thing of being here," Eavis added.
According to People, Bonas and Harry have been dating for about a year but their romance heated up earlier this year. In recent months, the pair have been seen skiing the Alps, partying in London and attending the June 22 wedding of friends Tom van Straubenzee and Melissa Percy, where Harry's ex, Chelsy Davy, was a bridesmaid.