Vienna is relatively – even remarkably – safe for a city approaching 2 million inhabitants.
This reputation reflects the country at large: the 2018 Global Peace Index ranks Austria as the third most peaceful country in the world.
In terms of Vienna itself, a 2019 report by Mercer ranked the city sixth in the world for “personal safety” (see below). And official statistics show crime in Vienna dropped over 10% in 2018 to a 19-year low.
Back in 2009, the European editor of Profil magazine was quoted as saying:
There are few cities in the world in which you can walk safely in the street after midnight. In Vienna, you can.
I’ve lived here 25 years and the only time I’ve felt personally threatened was when a rather dishevelled-looking man approached me on an empty tram and pulled what I thought was a knife from his jacket.
It wasn’t a knife, it was his ID and he was an undercover ticket inspector (for the record, I had a ticket).
More evidence for Vienna’s safety comes indirectly from its regular high rankings in global quality of life surveys. For example, it topped the 2019 Quality of Living rankings produced by Mercer (for the tenth time in a row).
This same report included a separate ranking for personal safety, which is where Vienna placed sixth in the world, one spot below Zurich.
Having said all that you should, of course, take the same care you would when travelling to any large city, particularly as regards parks and subway stations at night, and pickpockets in busy areas (particularly during the Christmas markets) and on packed trams and trains. Crime in Vienna is low, but there is still crime.
Consult your embassy or consulate for specific advice, especially concerning any current security issues. For example, see the security and safety advice for Austria provided by the US State Department, Australian Government and the UK Government.