The 25-year-old Australian actress ended up in a photo-booth with the royal and her showbiz friends Sienna Miller, Cara Delevingne and Suki Waterhouse - but at the time, Robbie was unaware of who he was, reported Female First.
"Everyone now thinks if they come on a night out with me in London they'll bump into Prince Harry," she said.
"But I didn't even know who it was. I'd had a drink or drinks. I thought I was hanging out with Ed Sheeran. I was thinking 'Ed's a really cool guy'. I'm going to take some pictures with him," Robbie said.