Do I need to validate train tickets that were purchased online?

There is a slight confusion in the question that I intend to clear up:

I heard that, in German cities, train tickets need to be validated in a machine; will I be required to do this for an online ticket? If yes, where can I validate.

This statement is only (generally) true for tickets

• bought at a ticket machine or a bus driver
• that are valid only within the city’s (in some select cases: cities’) transport association; and
• that do not already have a begin of validity printed on them.

If you have a ticket that requires validation, it will typically state so on the ticket; e.g. ‘Hier entwerten’ (with an arrow showing the direction to put the ticket into the validation machine).

Tickets for long-distance trains (e.g. ICE) and those bought online are never validated. They are printed out on A4 paper which is far too large to fit into the validator of any German city.

It depends on what type of service it is. For local services such as the U-Bahn or the S-Bahn, ticket validation machines are typically located at the station. These machines will stamp the time the ticket is validated, as tickets generally allow for unlimited travel for a defined region and period (e.g. 2 hours) after validation.

For intercity tickets, they contain a QR code. A conductor with a QR code scanner will check the tickets on board with a scanner. You will need to print out the ticket with the QR code so that you can present it (along with ID such as a passport) to the conductor.

As you've purchased tickets online between different cities, I suspect the latter.