With one short answer: YES!
Not only safe to drink, but really high quality as well. We have this luck In Ljubljana that groundwater doesn't have to be treated with any technical water preparation procedures, and is only chlorinated on rare occasions.
From 5 main water supply facilities and few local ones it than runs through some of 1.100 km of water network to the end user. It doesn't stay in the system for more than few hours, so your glass of water will always be fresh :)
In most cases you will be served a glass of tap water when ordering coffee in the cafes of Ljubljana without asking for it. Having a dinner in the restaurants and tasting some fine Slovenian wines, get a litre of tap water as well. Your head will appreciate it the next day.
We are proud to have almost 40 free taps with drinking water installed all over the town to help you fight dehydration. They are active in warmer months and regularly tested for quality. That's why we encourage you to refill your water bottles. It also means less plastic waste in our environment! We pass a few of this fountains also on our guided bike tours.
Don't know where the water fountains are located? No worries, simply download free app TAP WATER LJUBLJANA. It shows you the locations of fountains and directs you to the nearest one.
First water system we got during the period of Roman Ljubljana - Emona. Drinking water supply was coming from nearby hills. Later medieval times were culturally and hygienically definitely a step back. Residents had to get water from the wells which was also much less convenient.
It wasn't until late 19th century that the city council decided it is time for a proper water supply system. Man in charge of this important project was Ivan Hribar, who later became one of the most influential mayors of Ljubljana. Designer and constructor of the actual waterworks was Oskar Smreker. He was a well respected engineer that planned water systems of cities like Trieste, Belgrade, Bologna, Berlin, Mainz and Prague. In 1890 water finally started to run through pipes to the first 606 houses of Ljubljana!
Zgodovina hidrogeologije: kratek oris življenja in dela Oskarja Smrekerja (1854-1935)