The Lahngau was a medieval territory comprising the middle and lower Lahn River valley in the current German states of Hesse and (partially) Rhineland-Palatinate. The traditional names of the Gau are Loganahe Pagus or Pagus Logenensis.
The Lahngau was the East Frankish ancestral homeland of the Conradines. It was divided in ca. 900 into the Upper and Lower Lahngau (translated from the German Oberlahngau and Niederlahngau [or Unterlahngau], respectively).