His caption read, "Most elegant princess in the world."
It perhaps brought back fond memories for the troubled star. The former couple made one of their first appearances together at the same party back in 2011.
The post has come as a surprise to many - the couple were not thought to be on good terms. They were last seen together in January riding Segways and that same month shared a photograph of them embarking, which Justin captioned, "Love the way you look at me."
But Selena admitted that they had gone through a "bad breakup" during an interview featured in the March issue of Seventeen magazine.
"This year is when I really started feeling powerful," the pretty brunette said. "I had always been the underdog secretly.
"I was on Disney when it was Miley (Cyrus), the Jonas Brothers and Demi (Lovato). I questioned the way I looked, acted, sang and wrote."
She then revealed she had had a nasty split from Justin, who was arrested for DUI in Miami in February.
"Then I turned 20, went though a really bad breakup, and realised I needed to have Come & Get It be amazing. I wanted it all to be me."
She continued, "I actually spoke to Katy Perry about guys who are intimidated by strong girls. I haven't found someone yet who could understand my lifestyle, support it, love me through it, and not be threatened by it.
"It's hard. I do think I might scare some guys, because I'm like, 'I want to change the world! I have dreams! What do you want to do?' But I only know how to be me."