The word Bad (English: bath) is normally used as a prefix (Bad Vilbel) or a suffix (Marienbad, Wiesbaden) to denote the town in question is a spa town.
In any case, Bad as a prefix is an official designation and requires governmental authorization (which may also be suspended if a town fails to maintain the required standards).
The word Kurort is also used, meaning a place for a cure.
This list is alphabetical, the states of the spa towns are added, as well as their official German category designation (Heilbad, Seebad etc.).
Media related to Spa towns in Germany at Wikimedia Commons