Saint-Justin is a commune in the Landes department in Nouvelle-Aquitaine in southwestern France. In 1280, Constance Viscountess de Marsan …Saint-Justin is a commune in the Landes department in Nouvelle-Aquitaine in southwestern France. In 1280, Constance Viscountess de Marsan signed an “acte de paréage” with the Order of chivalry of the Hospitallers of St. John of Jerusalem, with which they founded the new Bastide of Saint-Justin. This agreement was signed without the authorization of Edward I, King of England, then Duke of Aquitaine.