Otsu-jo Castle, a "hira-jiro" (castle built on level ground) lay on the area around present-day JR West Otsu Station in Hamaotsu, Otsu City, Shiga Prefecture, and the "honmaru" (keep and main areas) of the castle was located near the existing pier in Otsu Port of Lake Biwa.
現在の滋賀県大津市浜大津、西日本旅客鉄道大津駅周辺一帯にあった平城で、本丸は、現在の琵琶湖大津港桟橋付近にあたる。