The short answer is you can, but you don’t have to.
The Staatsoper (state opera house) states in its house rules:
…persons dressed very informally (for instance, wearing short pants or leisurewear) may be refused admission
But getting through the door is not your only worry when it comes to clothes…
If you’re wearing jeans and a clean t-shirt you (probably) won’t get thrown out and you certainly won’t be alone, either.
Having said that, you won’t be viewed with unbridled joy by some of the better-dressed visitors.
My advice: simply dress elegantly as if you were going for a nice evening out to a fairly posh restaurant and don’t worry about it.
As a man, I usually wear decent trousers and a shirt with a collar, sometimes a sports jacket (but no tie) and I have yet to get any disapproving looks. For women a nice blouse/blazer, perhaps. Whatever you feel comfortable in, but respects the operatic ambiance.
- If you want to pull out all the stops, by all means put on an evening gown or dinner jacket. You won’t be alone and a night at the opera is, after all, a night at the opera. Drinking a glass of sparkling wine in the interval while dressed up in the magnificent surrounds of the Staatsoper makes for a great Viennese experience.
- If you are in the standing sections, expectations of dress are lower than for the seats. However, respectful clothing is still required, which means no tank tops, shorts or similar. If you wouldn’t wear it to visit a church in Rome, don’t wear it to the opera in Vienna.
- If you are lucky enough to be at a premiere or other one-off occasion, then dress up for it: expectations and requirements are higher.