The student took a bathroom break in an alleyway shortly before 3am on a February morning, Il Secolo XIX reported on Wednesday.
The 19-year-old had just left a local bar with some friends, and claimed he was unable to find any open public toilets or bars with the necessary facilities, forcing him to relieve himself in a secluded street.
Police caught him in the act and charged him with acts of public indecency - but it wasn't until the following week that the teen discovered the exact amount he was being fined: €10,000.
"I re-read the fine three of four times, thinking I'd misunderstood. But it was all true," the boy's father told Il Secolo XIX. "I sought legal advice but they told me it was better to pay; that we could appeal but the chance of winning was basically zero."
There was some consolation for the teenager, though, who only ended up having to pay a third of the total figure because he was able to pay straight away.
In early 2016, Italy de-criminalized acts of public indecency, which include not only public urination but also carrying out sexual acts in public, for example.
This meant that those found guilty now face administrative sanctions rather than criminal charges; in other words, no jail time, but a larger fine of between €5,000 and €10,000.
But the legal change may also mean that those caught short are less likely to be let off. In 2013, before the change, a 55-year-old was acquitted and let off a fine for public urination after Italy's Supreme Court deemed the act to be "urgent".
If the courts don't decide in your favour, peeing in public can turn out to be very costly indeed.
Last year, an Italian teacher was fired over a previous conviction for urinating in a bush. Though the offence itself was minor, failing to declare a criminal record is a sackable offence for teachers, and Italy's Court of Audit told the school the man had to be let go.
And sometimes it's not the act itself but the location which causes offence.
One man caught urinating on a Florence church - the burial ground of Renaissance artist Michelangelo - justified his act by saying he was an atheist.
This came just months after local taxi drivers had complained about tourists relieving themselves in the heart of the historic city.