The email was sent to medical students at the Free University of Brussels (ULB).
"From an aesthetic point of view," it read, " it is better for young women to wear a dress or skirt, and a nice revealing neckline."
Men were told simply to wear a suit for the event next month.
"Of course, ladies, this advice is not obligatory," the email added.
But mandatory or not, the suggestion caused an outcry on social media.
The university later released a statement on Facebook and Twitter, on behalf of its medical faculty, to apologise to all students who had received the message.
The advice was inappropriate and went against the university's values, it said.
A screen grab of the original email had been shared on a light-hearted Facebook page, ULB Confessions, which interrupted its usual stream of jokes and memes to express outrage at the "little scandal".
Hundreds of students responded, accusing the university of being sexist.
The ULB Confessions page also published a response it had received from a student: "No one has the right to tell you how you should feel in your skin. Nobody has the right to tell you how to dress. No one has the right to tell you how to play your role as a woman. No one has the right to take away this freedom that has been (and is still being) obtained with such difficulty."
The email was most likely sent by a woman as the secretariat did not include any men, the dean of the medicine faculty, Marco Schetgen, told Belgian news outlet RTL.
"No, it's not a hoax," he said, confirming the email had been sent out but adding that it had shocked him too when he found out.
A Belgian university has apologised after a message was sent to female students asking them to wear a low-cut outfit to a graduation ceremony.