Donauwörth is on the A7 motorway, the main north-south artery through this part of Bavaria, and is straightforwardly reached by car from either Augsburg (around 45 km south) or Nördlingen (around 35 km north). The B2 national road also passes through, connecting it to Augsburg and the Munich area.
By train, the town sits on the regional line between Augsburg and Treuchtlingen, with onward connections to Nördlingen via a branch line. Journey time from Augsburg Hauptbahnhof is roughly 30 minutes on a direct regional train. From Munich, the simplest route is to change at Augsburg; allow around 90 minutes total. There is no direct ICE service, but the regional connections are frequent enough that Donauwörth works as a day trip from Munich or Augsburg or as an overnight stop between the two.
The Romantic Road bus (Romantische Straße Busreise) stops here in season, running between Füssen in the south and Rothenburg ob der Tauber or Würzburg in the north. This is the most atmospheric way to link the Romantic Road towns if you are making a dedicated tour of the route, though the schedule is inflexible and the journey times are slow.