Do You Need a Passport from Vienna to Bratislava?

No. There is no passport control between Vienna and Bratislava. Austria and Slovakia are both in the Schengen area, so the train does not stop at the frontier, nobody boards to check documents, and nothing gets stamped.

Carry photo ID anyway

The absence of a checkpoint is not the absence of a rule. Both countries require you to be able to identify yourself to police on request, and hotels ask for identification at check-in. For EU, EEA and Swiss citizens a national ID card covers it. Everyone else should carry their passport — not because of the border, but because it is the only document you have that works.

Non-EU visitors: the 90/180 rule

US, Canadian, Australian, UK and many other passport holders can enter the Schengen area without a visa for up to 90 days in any 180-day period. Crossing from Austria into Slovakia does not restart that clock or add a new allowance — Schengen counts as one area, and a day trip to Bratislava is simply another day of the same allowance you are already using in Vienna.

The EU’s ETIAS travel authorisation for visa-exempt visitors has been announced with several start dates that then moved. Check its current status before you travel rather than relying on any article’s date, this one included.

Currency

Both countries use the euro, so there is nothing to change and no reason to hunt for an exchange desk at the station. Card acceptance in Bratislava is broad, and a small amount of cash is useful in the same places it is useful in Vienna.

What actually catches people out

Not documents — tickets. The Austrian ticket you bought covers you to Bratislava, but Bratislava’s city transport is separate unless you hold the EUR 19 Bratislava Ticket, which includes buses and trams on your first day. That is the more common day-trip mistake by a wide margin.