Was it the black/white/red, German Empire one? Or did it feature the swastika in some way?
The Hakenkreuzflagge, the symbol of the Nazi party, became Germany's official flag in 1935, including trade flag, to be flown by merchant men.
The Reichsflaggengesetz was enacted as part of the Nuremberg Laws (which included the racial purity laws) during the Reichsparteitag der Freiheit (Party Congress of Liberty) in, well, Nuremberg.
Source: http://www.documentarchiv.de/ns/1935/flaggen1935_ges.html (German language, and I cannot find an English translation, bit Artikel 2 determined the flag, Artikel 1 the colors)